Dealing damage (and taking it, with Damage Drive equipped) gives you Drive, which you can expend on the Forms or Summons. The Forms take differing amounts of Drive (Valor and Wisdom are 3, Master and Limit are 4, Final is 5) and take allies out of the fight (Valor takes Goofy, Wisdom takes Donald, Master and Final take both your current allies, Limit takes no one). Each Drive has seven levels, their own experience mechanics (Valor gets it from hitting things, Wisdom from killing Heartless, Master from Drive Orbs, Final from killing Nobodies, and Limit from using its Limits), and bonus abilities, including a Growth ability that increases in level as the Form does.

While in a Drive, you don't have access to your normal Action abilities, Limits (save in Limit Form), and only the Growth ability associated with the Form. Each time you Drive, you get Anti points, which increases your chance of Driving into Anti Form by accident. I don't know the particular mechanics, but fighting Nobodies increases the chances, and you will never Drive into Anti when Driving into Final (unless you're wielding Two Become One, which is the point of the keyblade). Driving fully restores HP and MP.

Anti is pretty strong, but doesn't benefit from keyblades, has no Guard, no Growth abilities, and no magic. It's a penalty, so it's understandable it would be bad.

Valor is only good as an early game power boost. No Guard, no magic, parryable melee hits and the modifiers of most of its finishers are 3.0. The only really nice thing is the early finisher with square.

Wisdom is a nice alt for Base. It replaces physical attacks with Shot, that, while weak individually, is long ranged, unparryable, you can move while using it, and pierces enemies, making it highly spammable and capable of killing faster in some situations. Wisdom makes magic faster and lets you move while casting (Fire becomes great), and increases the power of finishers. No Guard.

Master has overly long combos that are difficult to complete against anything, but very nice finishers (Master Strike has roughly a 5.4 modifier, very nice), the only ones in the game that will repeat the same finisher based on the enemy. All its physical attacks count as aerial hits, so Pumpkin is useless in it. No Guard. Master's magic disables finishers, but lets you chain normal magic hits until your MP runs out. Wisdom has as good or better magic in most cases, save for Master Thunder, which trades range for power (6.0). The meh magic, impractical combos and reduced safety leads to Master ranking lower than Wisdom in most cases, though it does have one notable use. You can use Master Thunder without breaking your string, even during finishers, and with the right set up (and lots of Shockcharm+'s) this can deal absurd damage: http://www.youtube.com/watch?v=MN6f5d3SDok&list=UUBkTWDE2JZwRl-y9_sEKpnA

Limit is considered the best Drive along with Final. The air strings are the worst in the game (even bellow Valor's), and the normal hits on the ground aren't very strong, but it boasts the strongest physical ground finisher in Zantetsuken (6.0). Ripple Drive is a nice AoE follow up with launch properties (2.25 modifier), has a better Guard that replaces normal Counterguard with Zantetsuken, and Dodge Roll as the Growth ability. You can't use magic, but instead, have the KH1 Limits shortcutted (Strike Raid, Sonic Blade, Ars Arcanum, and Ragnarok) which are quite strong and refill HP with every hit, though I believe you aren't invincible after the first part of Ars Arcanum.

While Limit offers utility and safety, Final offers unparralled power. The ground combos are awkward, the normal Blizzard hit isn't any better than Base's, the Thunder isn't for single targets, no Guard, and the worst Magnet in the game (no extra hits from smacking enemies in the AoE). However, the ground combos are still very strong, all it's normal hits count as finishers for bosses and are unparryable, and the keys attack automatically during certain actions (jumping, using an item, Gliding). The aerial attacks are the strongest physical attacks in the game, with Final Strike having a whopping 10.0 modifier.

The rest of the magic is absurd. Like Wisdom, you can move while casting. It has the strongest Blizzard finisher, Thunder hits everything, and Final Firaga is stupid. It has 9.6 modifier, is incredibly fast, spammable, has a vortex affect (drawing things closer) and high stun. One of the very best, if not the best, DPS attacks in the game. Just stick with the normal hit (the finisher is slower, usually doesn't connect with everything, and knocks things back; this is why Sleeping Lion can be nice with Final). Final Arcanum and Final Strike are unique as two part finishers; if you press attack or try to use magic during the first half, you'll cancel the second part, though you can take advantage of this with Final Strike and Fire:

Compared to the Forms, Base is a better meleer than Valor, Wisdom, or Master by midgame, and in some ways better than Final and Limit. Guard Break has 2.5 modifier, Aerial Finish a 3.5. Explosion has a 3.75 modifier, AoE knockback, and you're invincible during it. Magnet Splash is a 2.0, but gathers all enemies into the AoE *which is quite large* for a Magnet like stun before spitting them out, and you're invincible during it. Slide Dash is awesome. Finishing Leap is a 2.5 that launches you and the enemy into the air, opening them up to Horizontal Slash, the strongest spammable normal physical hit in the game (2.5). Horizontal Slash+Berserk Charge is hilarious. The magic is quite good, and Base has Guard and all Growth abilities.