Okay, as far as the base classes go:
Cleric: Full spell casting progression. If you're playing an evil cleric, you will be able to convert any of your non-domain prepared spells into inflict wounds spells, which means you touch an enemy and they take Xd8 worth of damage. You'll probably be expected to be the party healer, which means you'll have to memorize healing spells. Clerics have decent hit points and can wear heavy armor, which means in theory they can enter into combat, and get some pretty cool spells. Take a look at the cleric spell lists for more info. Plus, they get domains, which grant them extra abilities and allow them to take one domain spell for each spell level; domain spells often come off the arcane casting list, so you can have a cleric who might know teleport or something like that.
Sorcerer: Very similar to the wizard. Instead of having access to whatever spells are in your spellbook (and having the ability to add more spells to your spellbook as you go), a sorcerer knows a fixed list of spells that they can cast a higher number of times per day. For example: a first level wizard might have five first level spells in his spellbook. He memorizes three of them for that day, and once those are cast, he's done. A sorcerer only knows two first level spells, but he can cast those spells five times a day. (These numbers are pulled completely out of the clear blue sky, they're not really accurate). Sorcerers are based on Charisma rather than Intelligence, and get a slightly different set of skills, iirc.
Druid: Full spell progression, decent hit points, and the ability to turn into animals. They have a decent selection of divine spells, and their wild shape ability makes them pretty useful in combat.
The party you've described is lacking in both a primary healer AND a primary arcane caster, which is a problem. Basically, you need to decide if you want to be able to provide healing for the party (if you don't, you're all going to be spending cash on potions/scrolls/wands), or if you want the incredible piles of useful spells that an arcane caster can bring. I'd suggest going with the wizard-- you seem more interested in it at this point, and you've got a couple divine-ish casters already.