Major
These are seperate from the rest, because I believe they are more important than all the others. Otherwise, these Tags would go into the "Other" category. Seriously though, a class with one of these Tags has something different about them, for better or for worse, which a DM will likely need to take into account.
Gamechanger: These classes change an aspect of the game for the player who plays this class. Something becomes very different from normal, typically at a very low level, which makes playing this class fundamentally different from the rest. In many cases, this could be a seemingly small change and it may not give a major advantage or disadvantage, it just means that this class plays differently from normal, which might make from some very interesting play! Just don't mistake "interesting" for "good". The two aren't mutually exclusive, but they definitely aren't the same.
Pathfinder: This Tag should be obvious what it means. In case it isn't: it means the class was made for Pathfinder, rather than for D&D 3.5, so there are a few small differences, which are easily changed if you want to convert it.
Genre
The standard genre of D&D campaigns is Fantasy. Specifically, High Fantasy in a setting that resembles Europe around the year 1500, at the end of the Middle Ages and the beginning of the Renaissance. Most Homebrew fits this genre as well or easily can with some tweaks (to either the fluff of the class or to the campaign setting). Some classes are made to better fit a different era or location, however. Not every class has a Genre tag.
Horror: While Horror does not exclude Fantasy, Horror campaigns are a brand apart. They have a very different feel to them than “normal” campaigns do, which classes with this tag fit very well. Note that a lot of classes can be made to fit a Horror campaign and that these classes simply fit Horror campaigns amazingly well, often being specifically made for them.
Modern: If a class is based around gunmanship or was inspired by something set in the modern era (meaning post-1800), it will most likely receive this tag.
Oriental: The Oriental tag denotes that a class has fluff written with a Far East or (more rarely) Middle East setting in mind, be it Edo Japan, Han China or even the Mughal Empire of India, Sassanid Persian Empire or early Ottoman Empire. Note that a lot of classes that may be hit by the “Japan” tag (see below) might not have the Oriental tag (and vice versa). These classes may have the “Modern” tag (see above) instead.
Sci-Fi: A class tagged with Sci-Fi (short for Science Fiction, if you didn’t know) is likely a high-tech class and therefore out of place in a standard D&D setting. Some classes inspired by a sci-fi work might not be tagged with this tag due to not being particularly/exclusively Sci-Fi themselves (the Jedi from Star Wars being a good example).
Silly: These classes are a joke. Just kidding! Classes with this tag are most likely best-suited for a light-hearted campaign or
in a world where laughter was king. Note that with some refluffing these classes could still work for serious campaigns and you can still easily make a serious campaign with a party full of Silly classes (I bet some Batman villains had levels in Silly classes). Appropriately refluffed, these classes might even be among the greatest potential Horror classes.
Steampunk: Steampunk as a genre has really grown in the past decade and the Eberron setting has drawn from it as well (though less ‘steam’ and more ‘magicpunk’). Steampunk is best described as a world based off Victorian England (1800s era) with higher levels of technology that is steam-based, up to and including steam-powered mecha even.
Inspiration
The Inspiration group of tags describes sources from which the class drew inspiration from and may be entirely based off. Not every class has an Inspiration tag.
Book: If a class is based off something from a novel or a book series, it will have this tag. A Harry Potter style wizard would be a prime example.
Comic: The class was based off something or someone from a comic book. This is often the case with superhero-themed classes. Though manga are comic books, they instead fall under the “Japan” tag (see below).
Historical: The class was inspired by something from real-world history. Seems like these would be great for low-magic campaigns, I imagine.
Japan: Everything based off Japanese television, anime or manga falls under this. Note that a lot of classes that may be hit by the “Oriental” tag (see above) might not have the Japan tag (and vice versa).
Original: If the class wasn’t seemingly inspired by any other media named here, it might get the Original tag. Classes that have the Original tag are usually rather different in fluff or execution of mechanics from other classes. They are never just clerics, fighters, rogues or wizards. This tag isn’t handed out easily!
Television: If the class is based off a movie or a television series and doesn’t fall under the Japan tag, it gets this tag.
Video Game: The class was clearly based off or inspired by a video game and may or may not explicitly call out to that by name alone.
Subsystem
Some classes make use of their own special mechanic. Some classes are just bland. Some classes make use of a specific magic subsystem of D&D that some other classes might also use. These are some of them. Remember that to use a class that uses a subsystem you will have to learn the subsystem as well, which brings a steeper learning curve with it.
Arcane: The class is an arcane caster, like the wizard, sorcerer or warlock. They may be prepared or spontaneous and sometimes know their full spell list. They need not be vancian casters, as with invokers.
Binding: The class binds vestiges to themselves, like the Binder from Tome of Magic.
Divine: The class is a divine caster, like the cleric or shugenja. They may be prepared, in which case they might or might not know their full spell list, or spontaneous. They need not be vancian casters, as with invokers, though divine invokers are more rare than arcane ones.
Incarnum: The class uses incarnum, thus having the power of Soul on their side. See Magic of Incarnum for more details on the subsystem.
Infusions: A rarity, even in Homebrew, infusions are the purview of the Artificer from the Eberron Campaign Setting. This class is stealing his job! Which might be a good or a bad thing, depending on how it’s handled, seeing as the Artificer is Tier 1.
Invoker: At-will spell-like abilities are your thing! Invokers don’t really need sleep, which gives this class bonus points as well. The only official invokers are the Warlock from Complete Arcane and the Dragonfire Adept from Dragon Magic.
Martial Adept: The class is a martial adept and learns maneuvers from martial disciplines, like the classes from Tome of Battle. You’re in luck, as there’s plenty of Homebrew support for this kind of character.
Psionic: The class is a psionic manifester, like the classes in the Expanded Psionics Handbook. Lucky for you, they’re also in the SRD, making a psionic class easy to fit into a campaign even if the DM has no access to books at all.
Shadow: The class uses Shadow Magic, like the Shadowcaster from Tome of Magic. If you’re playing in the Forgotten Realms, be sure to stay on Shar’s good side.
Truenames: The class uses truenaming, like the Truenamer from Tome of Magic. It probably uses a Homebrew fix.
Unique: The class uses a unique Homebrew system that either the class’ creator or someone else made. These are often Big Reads, but the ones that aren't are often incredible.
-Unique: This class does not use a unique Homebrew system, but makes a new and unique twist on an existing subsystem, such as using Incarnum for something other than soulmelds, or using power points to fuel class abilities rather than manifest powers.
Vancian: The class uses Vancian casting, the same sort of spellcasting system the cleric, druid and wizard each also use and which is the standard spellcasting system for D&D.
Role
Party roles are important in a party, that’s just how they roll. Like dice. You like dice, don’t you? Long story short, parties function better if there isn’t too much overlap in roles and they complement one another, whether by the same, similar or totally different roles that support one another. When you’re late to the party, don’t fret! It may seem like you’re “stuck” with a role, but you could easily help them in some other way or find a way to make that role still fun for you. When life gives you lemons, you can either make lemonade or engineer a combustible lemon to burn life’s house down with. Your choice.
Buffer: You grant your allies bonuses or minor abilities, making you a great support class. Awesome! More people should be like you, because friendship is magic.
-Self: The -Self Tag is a sub-Tag added only to the Buffer Tag. This designates that the class is a Buffer that mainly buffs themselves, or maybe they're not even capable of buffing others.
Crafter: While some campaigns don’t give you enough downtime to do any crafting, the Dedicated Wright could help you very much in that regard. If you can’t afford to buy or make one and won’t have a lot of downtime, either ask your DM for some downtime or don’t focus on crafting. If before the campaign started it was clear you might be on an in-game schedule, don’t focus on crafting. Otherwise, crafting can be extremely rewarding for both you and your party, saving you tons of gold pieces, whether you craft mundane or magic gear.
Debuffer: The evil opposite of a buffer (often even literally), the debuffer bestows penalties on enemies rather than bonuses on allies. Debuffers are often controllers, limiting the opponent's movement and actions.
Face: A party Face is someone who is good at dealing with social situations. While this might sometimes be partially dependant on the player’s own roleplaying abilities, this class is just geared up for it. They’re often Charisma-focused, have Bluff, Diplomacy/Intimidate and Sense Motive as class skills and have class features or spells that make making people friendly easier. This role is almost never taken as a character’s only role, except in low-op games.
Healer: Often it is said that a party doesn’t need a dedicated healer in 3.5 campaigns. Either way, a Healer can still be pretty darn handy. Plus, this guy may very well be capable of other stuff, especially if he still managed to be Tier 3 or above.
Magic: You use magic, plain and simple. If you use a subsystem, you’re most likely using a magic class here, but that isn’t necessarily so. Beware anti-magic fields, for they shut you down, even if you mostly just use supernatural abilities.
Melee: You get into the thick of things and will have to stand within 10 ft of your enemies most of the time to deal damage.
Moneymaker: Lemme tell you, this Tag is typically
rare. The Moneymaker Tag refers to classes who can earn a lot of money. It can easily go hand-in-hand with the Crafter tag, but can also be applied to merchants, performers, gamblers (the lucky ones) and classes that somehow manage to trick out the Profession skill.
Ranged: You will probably not want to attack if you are within 5 ft of your enemies, so stay back and let the melee get up close instead. Use your bow or your magic to deal damage or control the battlefield from afar. If a class has neither the melee nor ranged tags, it will likely want to stay away from melee if it has the “Magic” tag, otherwise it is possible it can do both Melee and Ranged, but isn't particularly made for either.
Sage: The Sage Tag denotes a class that can most likely trick out the Knowledge skills. Sage classes tend to have all Knowledge skills as class skills or otherwise at least a sizeable amount. They often get class features that rely on Knowledge checks or make them better, though this is not necessary. Sage classes often also have Decipher Script as a class skill and they are never illiterate.
Ever.
Scout: Most likely is a class with plenty of skill points, the Listen and Spot skills, often the Hide and Move Silently skills, plus also often abilities that make them better at detecting something. Movement options are typically very important for a good scout. Scouts are often good options for wilderness survival games or for tracking as well, as half the time they have Survival on their class skill list.
Tank: A rarity in D&D, a tank is a character who goads enemies into attacking him rather than his allies or otherwise prevents his allies from behind harmed. These classes can almost always be found in melee or six feet under.
Trapmonkey: "It's a trap!" is likely a line you might often find yourself saying with this class. A trapmonkey is a class that usually has trapfinding or otherwise deals with traps, disarming them or setting them for others. In most campaigns, a dedicated trapmonkey is unnecessary, but they can be handy in dungeons.
Special
These tags don’t really fit with the others and not even with each other, really.
Big Read: A handy tag for DMs, this tag denotes a class for which you will most likely need to read a lot. Players, know that most DMs who allow Homebrew sparingly will be less likely to allow a class that is a Big Read! Exceptions always exist, especially among DMs, but a warning is in order.
Expanded: The base class has garnered a modicum of popularity or simply has a very dedicated creator, causing it to have Prestige Classes attached to its name. If the class is tied to a Homebrew subsystem, there may also be other base classes made for it.
Fix: While most Fix classes will be listed in the fourth post of this thread, some may be listed in the second post with the Fix Tag instead. This is because these classes were intended to be Fix classes, but they are very much different from the original official class, which is why I put them with the other ones instead.
Incomplete: The class is incomplete, but it is definitely playable. Classes with this tag might not have all their levels finished and be playable up to a certain level or be made to allow for a character to take certain paths within the class, but not all paths are done yet, making your options more limited. Don’t see this as a bad thing! The class usually only drops half a Tier if it doesn’t have all options worked out and if not all levels are done you can still multiclass or prestige out. Note that very few incomplete classes will be listed and these few, these happy few, are an exception to the “complete classes only” rule.
Roleplay: Classes with this Tag become even better in campaigns that are roleplay-heavy with less actual combat or even no combat at all. Generally, all Tier 1 and Tier 2 classes could have this Tag by default, but that is because their great levels of power become even more of an asset in roleplay-heavy campaigns, as others of lower Tiers become even paler in comparison. As such, this Tag is reserved for classes that actually deserve it.