Basic 3.5 D&D classes: Slightly organized.

--Martials: People whos job is mostly to fight in melee combat.
Fighter: A pure martial, with LOTS of feats and all proficiencies.
Barbarian: A big, angry martial that rages and is illiterate.
Monk: A martial who fights unarmed and unarmored. Kind of sub-par even for martials.
Paladin: A heroic divine martial who has healing powers, and later on gets slight amounts of cleric magic.
Ranger: An archery-or-duel-wielding martial who, later on, gets slight amount of druid magic.
Dragon Shaman: A kinda-tough martial with some dragonfire abilities, and the ability to buff allies.
Marshal: A martial who really, really buffs allies. Wants a lot of charisma for maximum benefit.
Knight: A chivalrous warrior who's all about challenging the foe headon and protecting his allies.
Ninja: A sneaky cross between a rogue and a monk. Ki abilities, sneak attack, the like.
Scout: A strange cross between a fighter and a ranger, without the divine magic. Good at wandering and sneaking around, but awesome at fighting too. Amazing with a bow.
Duskblade: A hybrid between a caster and a fighter. Specializes in damage-dealing and buff spells, and can cast spells *through* his attacks.
Hexblade: An 'arcane' version of a paladin, except slightly worse. Has 'curses' and stuff, but most of the case they aren't very useful.
Samurai: A REALLY BAD martial. It's bad. Gets a few abilities that are basically fighter feats, except he gets less of them than the fighter gets feats. Not worth it.
Swashbuckler: A light-armored dodgy martial that's really good with a rapier.
Psychic Warrior: A hybrid fighter/psion. Pretty simple, but with the right powers can be really powerful.
Soulknife: A psionic martial who uses his psychic powers to generate 'soulknives', psychic blades that they can upgrade and throw around. Never actually uses psionic powers. Your blade is your SOUL. Infinite Blade Works, anyone?
Divine Mind: A hybrid ardent/fighter, who starts out with just the buffs and can spread those buffs to his allies.

--Skillmonkey: Diplomancers, trapfinders, pick-lockers(or -pocketers), and jacks-of-all-trades. Less about the combat and spellcasting, more about everything else.
Bard: A jack-of-all-trades with a bit of arcane magic, a bit of fighting skill, a bit of skillmonkery. Buffs allies.
Rogue: A complete skillmonkey, with sneak attack and awesome dodge abilities.
Beguiler: A hybrid between a caster and a rogue. Specializes in enchantment, illusion, and transmutation spells. Good at being sneaky.
Factotum: Everything. The Factotum has 'inspiration' points with which he uses to emulate nearly every major class feature. Can be a fighter one day, an arcane caster another, etc.

--Incarnum Classes: Incarnum classes take soulstuff and put them into 'soulmelds' every morning that give them buffs. Can also put in and take out 'essence', which upgrades the soulmeld accordingly.
Incarnate: An 'incarnum' class who builds pseudo-magic items called 'soulmelds' and fuels them with 'essence'.
Totemist: A much better martial incarnum hybrid where the totemist has unique, nature-themed soulmelds and a 'totem chakra' to bind things to. Can get really scary if you want to fight in melee.
Soulborn: A really shitty martial incarnum hybrid that's mediocre at both things.

--Maneuver Classes: Basically Martial classes but better, Maneuvers are special abilities you learn and use, similarly to spells, except they're all combat-based and most aren't explicitly magical. If you want to be a melee martial, be a maneuver class instead.
Crusader: A 'maneuver' martial class who is granted special attacks by the gods.
Swordsage: A lightly armored maneuver class who gets unique, 'sneaky' special attacks.
Warblade: A full-on martial maneuver class. Straightforward fighter, but can be really scary with the right maneuvers.

--Casters: People who cast spells. Usually pretty squishy, though druids and clerics are a lot tougher.
Cleric: A divine caster- god-themed, with different 'domains' they get access too. Also turns undead.
Druid: A divine caster- nature themed, with an animal companion and shapeshifting.
Sorcerer: An arcane caster who does his magic on 'instinct'. Less access to spells, but more spells-per-day.
Wizard: An arcane caster who studies to learn magic like a huge nerd. Gets access to tons of spells, but you have to 'prepare' each individual spell individually.
Healer: A divine caster who has primarily healing spells, and gains paladin-like 'lay on hands'. Just be a cleric or Favored Soul instead.
Warmage: A hybrid fighter/caster who is really good at casting blasty spells.
Wu jen: A strange arcane spellcaster who gets a unique spell list, but has 'taboos' they can't break without losing access to their spells for a while.
Favored Soul: Almost a sorcerous version of a cleric, with less spells-known but more spells-per-day. Gains benefits depending on which deity they follow.
Shugenja: Another 'spontaneous divine caster', the Shugenja is limited by how many 'elemental' and 'normal' spells they have.
Spirit Shaman: A druid that's more about ghosts and spirits instead of nature and shapeshifting.
Dread Necromancer: A necromantic caster who gets various touch abilities and slowly becomes a lich as they level.
Archivist: A wizard-themed divine caster who learns divine spells by research, the way a wizard can pick up every spell.

--Psionic Classes: A bit different than Arcane or Divine magic, Psionics runs on a 'mana-based' (Power Point) system, which powers even non-spells in some classes.
Psion: A psychic power using 'sorcerer', who uses mana to cast his spells. Specializes in a certain type of powers.
Wilder: Like a psion, with less powers, but has the ability to do a kind of magical 'barbarian rage' that improves their powers and makes them stronger, but at a cost.
Ardent: A psionic who adopts unique 'psychic mantles', which grant buffs and powers the way cleric domains work.
Erudite: A psionic 'wizard' who learns psychic powers, and can theoretically use every power.
Lurk: A hybrid psion/rogue, who is sneaky AND has powers.

--Invoker Classes: Technically arcane casters, Invokers have unlimited spell uses, but extremely limited spells.
Warlock: A spellcaster who's speciality is in having a few spells that they can use INFINITELY. Never runs out of spells. Also has 'eldritch blast', a damaging spell that scales and can be given different 'shapes' and 'essences'.
Dragonfire Adept: Like a warlock, except with different spells and the ability to breathe fire instead of shoot lasers.

--Other Classes: Ones that don't fit any standard category.
Shadowcaster: A somewhat broken caster who has barely any abilities, and can barely use them. Not extraordinary without the right feats or abilities.
Truenamer: An outright broken caster, who needs a lot of GM assistance to even work. Is a 'skill based' caster, where every roll decides if your 'truespeech' spell works or not.
Binder: A strange sometimes-martial who adopts 'vestiges' into his body, giving him unique suites of powers and abilities.
Artificier: An item-crafting class that also has pseudo-caster abilities, to attach buffs to items for a time through what's called 'infusions'.

--Generic Classes: Generic classes can pick their skills, pick their saves, and pick their class features (or take bonus feats instead of class features). Unearthed Arcana, so ask for GM permission.
Warrior: Generic class, fighter-style.
Expert: Generic class, skillmonkey-style.
Spellcaster: Generic class, sorcerer-style.