Hmm. So you're not familiar with psionics at all -- and perhaps Psionics Unleashed doesn't give a good introduction? Well, I can't help you specifically with Pathfinder psionics, but I can give you a primer in 3.5e psionics, so that the changelist you mention will mean more to you.
Psionic Classes and Powers
I'm going to assume that you're familiar with 3.5e spellcasting. Psionics is like that, but...
- You have "powers", not "spells", which you "manifest", not "cast". Powers are divided into "disciplines", not "schools".
- Psions are Int-based manifesters that must specialise in a discipline; Wilders are Cha-based manifesters that share most of the psion's power list, but manifest powers from innate talent, not study. However, this is as far as any comparison to wizards and sorcerers goes.
- The psychic warrior is the Wis-based manifester, but is somewhere between bard and ranger in its actual role. The soulknife is not a manifester and will be skipped here.
- All powers are spontaneous, not prepared, and manifesters are limited in their number of powers known.
- Powers have nine levels, just like spells (but no level 0), but manifesters do not have slots.
- Instead, manifesters have a number of power points, which can be spent on any power the manifester knows. A power costs power points equal to (twice its level, minus one) to manifest under normal conditions.
- Like spell slots, power points are fully replenished by a good night's rest.
- Powers generally do not have level-dependent effects like spells do. Instead, a manifester can augment the power by spending extra power points on it. The effects are specific to each power. Some powers have more than one augmentation; these can be "either/or" (but you can spend even more points to get both), or can be "all the points you spend to get that augmentation, also count towards this one".
- This is important: a manifester can spend no more than his/her manifester level in power points on a single power. The wilder has class features that let it break this limit (at some risk). There are also a few special cases where a negative effect can force a manifester to spend more power points than normal; these extra points are wasted and don't count against this limit.
- Powers do not have "components", they have "displays". That's like all spells being stilled and silenced, but flashing a big neon sign over the caster's head when used. However, a Concentration check can suppress all displays.
- Powers still provoke attacks of opportunity, however. Manifesters need to make Concentration checks, just like casters, to avoid these.
- By default, spells and powers interact with each other like they were the same thing, or their nearest equivalent. Powers are affected by spell resistance and dispel magic, and spells by power resistance and dispel psionics. There are few exceptions. This is called "psionics-magic transparency", and optional rules can be selected that change this default to a greater or lesser extent.
Psionic Feats and Skills
Any creature with power points, whether or not it's a manifester, is "psionic" and can take [Psionic] feats. (Creatures can have power points as a racial feature, or by taking a feat that gives them points.)
- Many psionic feats are built around something called "psionic focus". Any character with at least one power point still remaining can make a Concentration check to gain focus.
- Gaining focus is a full-round action (and so it's costly in the middle of combat). The feat Psionic Meditation reduces this to a much more practical move action.
- Some feats are active so long as the character is focused.
- Other feats are activated by "expending" the focus. Once done, the character must take another action, and make the Concentration check, to regain focus.
- Metapsionic feats (not "metamagic") are of the second variety (expend your focus). This is in addition to, not instead of, costing more power points (which is like using a higher spell slot -- +2 power points is like +1 spell level -- and don't forget the cap on power points spent per power). In return, they either have a smaller cost, or better effects than the equivalent metamagic feat (if one exists -- many have cool and unique effects, like Burrowing Power or Unconditional Power).
- Spell Focus and Spell Penetration also have their equivalents, which also require expending focus. In return, their effects are strictly better than their magic counterparts. Psionic Endowment boosts the DC of any power, not just one school/discipline. Power Penetration gives a much larger bonus (+4, or +8 for the Greater version).
- There are also bunches of cool and interesting feats that really don't have equivalents in the core rules.
- And then there are some that specifically say they violate psionics-magic transparency. Me, I don't really like those ones.
- On the skills front, Knowledge (Psionics) matches Knowledge (Arcana), Psicraft replaces Spellcraft, and Use Psionic Device fills in for Use Magic Device.
- There's also a brand new skill called Autohypnosis that lets you use skill checks in place of various saves and percentage chances, in a "mind over body" sort of deal.
Mostly, a creature's a creature, psionic or not. There's just one special rule, and two special types of creature, that you need to be aware of.
- Creatures can have psi-like abilities (Ps) instead of spell-like abilities (Sp). These cost no power points, and -- this is the important bit -- they're always augmented to the maximum that the creature's effective manifester level would allow.
- Astral constructs replace all the summon monster-type lists. They're built to order, from lists of optional abilities, whose availability depends on the augmentation of the power used to create them. (No astral construct I through IX; one power, increasing augmentation.)
- Psicrystals replace familiars. (Psionics and crystals have a thing for each other, in case you hadn't noticed.) A psicrystal is a tangible shard of personality and intellect, which is (or can be) self-mobile. It gives its master a skill or check bonus based not on its species (since it's not an animal), but on its personality.
- There are psionic weapons and armours which work more or less the same as their magical counterparts (special abilities costing the equivalent of +1 bonuses, that sort of thing). Psionic weapons don't shed light; they make noise. Most have crystal embedded in them, or are made from it.
- Instead of "wondrous items", psionics gives us "universal items" (universal 'cause anyone can wear and/or activate them). The variety and kinds are much the same -- amulets, boots, gloves, slotless items -- but they also introduce a few new categories, including "crystal masks" and "psychoactive skins".
- The one psionic ring in the basic psionics rules is a universal item. Anything comparable to a rod would be, too.
- Psionic tattoos are equivalent to magic potions. Yes, potions, not scrolls; their scribed nature gives them a superficial similarity to scrolls, but it's only skin deep. Like potions, they're limited to 3rd-level powers or below, and they're usable by anyone who has one, regardless of class. Tattooed powers vanish when used, which is like scrolls... but also like potions. And they're even portable like potions: psionic tattoos can be made to "get up and walk" to a different place on the wearer's body, or even to another creature.
- A cognizance crystal is unlike any magic item. It's simply a repository of power points that any manifester can use instead of their own pool.
- A dorje is a psionic wand, plain and simple. It's made of crystal (surprise, surprise).
- A power stone is the psionic equivalent of a scroll: can hold multiple powers; takes a Psicraft check to "decipher" (which involves "addressing" the power stone, in contrast with "reading" a scroll); chance of mishap if you're not high enough level to manifest that power. A power stone is a thumb-sized chunk of... crystal.
- Psicrowns are the equivalent of magic staffs, since they use their stored energy (their own power point pool) to manifest any of a specific list of powers, but the wielder's level and ability scores determine the parameters of the effect.
- The wielder's power point limit still applies when using cognizance crystals and psicrowns.
I'll have to go looking for an authoritative list of "the tiers" to be sure, but here's what I recall...
Psions and wilders are comparable to full casters, but I believe they sit a tier or two below them; they are very powerful, but not easily breakable like full casters are. There are thousands of spells a caster can choose from, but relatively few powers for a manifester, so there's fewer bad ideas that slipped past the playtesters. Additionally, the power point system has been hailed as effective and well balanced by many gamers.
The psychic warrior is similarly like the bard but less powerful. It's got up to 6th-level powers, but its natural focus is combat, not diplomancy.
The soulknife is not a manifester. It's a psionic class, but that only really means it can take psionic feats; in all other respects, its class features might just as well be magical, because they're all (Su) or (Ex). It's been called weak (on par with the monk, in fact), as it's a melee character with average BAB, light armour, and no exceptional powers to increase its damage (it has a few, limited abilities that are kind of rogue-like). Its main ability is also its biggest weakness: it has a magic weapon that it materialises out of nowhere. This weapon increases in power with class levels... so multiclassing or prestige classing is fatal to the soulknife's usefulness (except to soulknife-specific classes; the soulbow, I think it's called, is supposed to be good). The magic powers and even the type of weapon are also tightly controlled; no vorpal greatsword for you.
Still, I think it's a cool and flavourful class that could do well as a skirmisher in actual play, just like I've got a player who plays a monk, enjoys it, and contributes very strongly to the party.
Some say that psionics is broken because a psion can "go nova", augmenting even a low-level power with a large chunk of their power points, until it does colossal amounts of damage in one single devastating blow. This argument usually overlooks the "no more points than your manifester level on one power" limitation.
Some have bad memories of psionics before 3.5e and are hard to convince that it's actually good and balanced now. I don't know much about 3e psionics (and really nothing about 2e), but I do know it used an involved system of "attack modes" and "defence modes", with a table of specific interactions between them. I once read quite a succinct and convincing article that said, basically, "You wouldn't accept it if fighters were massively vulnerable to arrows if they'd held their shield in this way to ward off swords, would you? That's what attack and defence modes were like."