As ThirdWizard said. But to elaborate a bit:
1. THAC0 is replaced by Base Attack Bonus, and for an attack you just roll 1d20 + your BAB + any additional modifiers, and hit if you roll equal to or better than the opponent's AC. Base AC is still 10, but high AC is now desirable.
2. High levels in a class now grant rolled HP from hit dice, rather than a fixed number.
3. There are only three kinds of saving throw now, and you stack the base save bonuses from each class level you gain.
4. Multi-classing is now universal for all creatures, and resembles 2E dual-classing. However, when you multi-class you retain all your old abilities, suffer no special penalties/hindrances for using them, and can return to advancing in previous classes at any point.
5. You can only gain each feat once, unless it specifically says otherwise in its description.
6. Thief Skills are now just normal skills mixed in with the others, which are all sorta like non-weapon proficiencies from 2E. Any character can gain ranks in most thief-ish skills.
7. Arcane casters, such as wizards, sorcerers, and bards, can now cast their spells while armored. However, they suffer from an Arcane Spell Failure chance from armor they wear.
8. Clerics and druids do not suck, and are now the best classes. No, really, I'm not yanking your chain. Stop laughing, I'm really freaking serious here. Clerics are actually good to play in 3E/3.5E. Clerics can wield any weapon now, they just need to spend a feat on Martial or Exotic Weapon Proficiency if they want to be good with that weapon.
9. Characters are no longer limited in level nor class options due to race. Any race can take any class, and gain any number of levels in it. Play all the halfling wizards and gnome druids and dwarf paladin/enchanter/rogues you want, or whatever.
10. Magic Resistance is now Spell Resistance, but it still applies against all magical effects, for the most part. Spell Resistance is not a percentage, instead it's a specific number, and when you cast a spell or something on a monster with Spell Resistance, you need to roll higher than its SR in order to affect it; you roll 1d20 and add your caster level to the check.
11. Oh, and you don't get automatic double damage on a critical hit. You also don't get automatically get a crit on a natural 20. A natural 20 is still an automatic hit. On a crit, you roll extra times for damage, based on the weapon you use; the default is double damage on a crit, in which case you just roll damage twice and add the modifiers twice, but if you wield a battleaxe for instance, you instead deal triple damage on a crit. Also, your weapon determines how often you get a crit. Weapons have a 'threat range' which is the range of numbers you can get a crit on, with a natural d20 roll; for instance, longswords have a threat range of 19-20, so you threaten a critical hit if your attack roll is a natural 19 or natural 20, before modifiers. Instead of an auto-crit, however, you have to roll the attack a second time, and see if that second roll (after modifiers) equals or exceeds the target's AC. If so, then you get a critical hit, but if not, you just get a normal hit. However, if you roll less than a natural 20 on your normal attack roll, you don't threaten a crit unless your hit succeeds. So for instance, if you wield a longsword and hit the opponent with a natural 19, but happen to have an attack roll modifier of -3 (for instance, if you were hit by a Ray of Enfeeblement), the attack misses unless your opponent has an AC of 16 or lower. If you did hit then, though, you'd roll again because you had a natural 19 (which is in the longsword's threat range), and if your second roll would've hit the AC, you got a critical hit.
