Level

Level, simply put, is how powerful you are. The higher your level, the more damage you do with your magic and physical attacks, the less damage you take, the more Vitality you have, and the more spells you can cast before the MP gauge depletes.

To level up, you must gain experience. You gain experience by defeating enemies, unless the form or situation you are in requires a different method. Each game has different rewards to leveling up.

Leveling up
Here are all the rewards you can get for each individual game. This includes level up rewards for not just Sora, but also Donald and Goofy except where noted.

Kingdom Hearts

 * 1) Increase in Health Points.
 * 2) Increase in Magic Points.
 * 3) Increase in Strength.
 * 4) Increase in Defense.
 * 5) Increase in Ability Points
 * 6) A new ability.
 * 7) More accessory slots.
 * 8) More item slots.

Sora

 * 1) Increase in HP.
 * 2) Increase in Card Points.
 * 3) A new Sleight.

Riku

 * 1) Increase in HP
 * 2) Increase in Attack Points
 * 3) Increase in DP, or Darkness Points.

Kingdom Hearts II

 * 1) Increase in Strength.
 * 2) Increase in Defense.
 * 3) Increase in Magic (Only Sora and Donald)
 * 4) Increase in Ability Points
 * 5) A new ability.

Bonus Level
Bonus Level is a reward that is only in Kingdom Hearts II, because you gain no experience from bosses. You still get rewards after certain boss battles in Kingdom Hearts, but it isn't called Bonus Level. Sora, Donald, Goofy and (if applicable) a fourth party member all receive a Bonus Level after a boss fight in Kingdom Hearts II. All magic spells received are for the entire group, but some abilities learned are only for some characters.

You do not obtain Bonus Levels by gaining experience; rather, you increase it by defeating Bosses. Bonus levels function the same way as regular levels otherwise, gaining the aforementioned bonuses (pre-determined on each fight though), however there's a few more things you can gain:

Kingdom Hearts

 * 1) A new spell or upgrade to it.
 * 2) A new Ability.
 * 3) A new Accessory.

Kingdom Hearts II

 * 1) Increase in Health Points.
 * 2) Increase in Magic Points.
 * 3) More armor slots.
 * 4) More accessory slots.
 * 5) More item slots.
 * 6) A new spell or upgrade to it.
 * 7) An extra Drive Gauge. (Only Sora)

Form Level
Every form (except for AntiForm) has a separate level, which experience for is gained by different means than the regular form's "kill enemies" rule, and can only be gained if the conditions are fulfilled while in that form. At levels 3, 5, and 7, you get an upgrade to that Form's Growth ability and to Sora's growth ability corresponding to the same form (at level 3 you gain it, 5 and 7 upgrade it). Forms can have Growth abilities up to Max Level, and regular Sora can only have it up to level 3.

You also gain an ability for Sora when the Form levels up to levels 4 and 6, and is in most cases the only way to get them. In every level, you also gain a Form Gauge upgrade which increases the time a Drive Form can be used.

Kingdom Hearts
There is no way to level up your summons in Kingdom Hearts, but you can increase the amount of time the stay on the field by equipping the Cheer ability to Sora, Donald and Goofy.

Kingdom Hearts II
In Kingdom Hearts II, Summons can be leveled up by using any one of them. You earn one summon experience point for every Summon Gauge bar that is consumed during a summon. Once enough points are gained the Summon Gauge increases so summons can stay out longer.