What do the spirit, vitality, etc. stats affect?
Solution 1:
Spirit affects magic defense. Having more spirit lowers incoming magic attack damage.
One note from the wiki:
The player's overall Magic Defense was intended to be a sum of the character's Spirit stat and the MDef value of their armor, but due to a bug in the coding, the MDef value is not used and Spirit alone determines the character's Magic Defense.
Vitality affects physical defense. High vitality lowers damage from physical attacks.
EDIT: There's a full guide here (Thanks for the wiki link):
http://faqs.ign.com/articles/452/452504p1.html
Pasted relevant information:
?================?================?==========================?===============?
| Stat Name | Purpose | How Stat is Increased | Stat Maximum |
?================?================?==========================?===============?
| Strength | Attack Power | Power Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
| Dexterity | Speed (ATB) | Speed Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
| Vitality | Attack Defense | Guard Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
| Magic | Magic Power | Magic Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
| Spirit | Magic Defense | Mind Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
| Luck | Critical Hits | Luck Sources | 255 (+) |
| | | Gaining Levels | |
?----------------+----------------+--------------------------+---------------?
Solution 2:
I found another guide that goes into much more detail for each stat, from this guide:
HP (Hit Points):
HP is a character's life. When depleted, the character will be KO'd in battle. When the HP of all characters in your party is depleted, it will be game over. HP is increased when a character levels up.
MP (Magic Points):
MP is used to cast Magic. Every spell requires a certain amount of MP to be cast. The points are deducted after the casting. When you have insufficient MP to cast a certain spell, you simply can't it.
Strength (Str):
Strength is used to power up a character's physical attacks. The higher your Str, the higher damage you will do with physical attacks.
Vitality (Vit):
Vitality is used to power up a character's defense against physical attacks. The higher your Vit, the less damage you'll receive from enemy attacks that fall under physical attacks.
Magic (Mag):
Magic is used to power up the magic that a character can use. This includes summons as well as regular magic and enemy skills. The higher your Mag, the higher damage you'll hit with magic.
Spirit (Spr):
Spirit is used to power up a character's defense against magical attacks. The higher your Spr, the less damage you'll take from enemy attacks that fall under magical attacks. This is an important stat, because there is a bug in the game that causes the magical defense that armor gives not to be taken in account. This means your magic defense is always just your Spirit stat.
Dexterity (Dex):
Dexterity is basically the speed of your characters, and it determines how fast their bars will fill up. Dexterity is ALSO an important part of a characters evasion against physical attacks. Thus, it's part of Defense% (= Physical Evasion). The higher your Dex, the faster your character will be, and the more physical attacks they will evade.
Luck (Lck):
Luck increases the chance to perform a critical hit with physical attacks. It ALSO increases the chance to get a 'lucky evade' in which case you will evade physical attacks. The higher your Lck, the more critical hits you will perform, and the more attacks you will evade. The chance formula for getting a lucky evade is like this: % = [Luck/4]. Max Luck (255) would give you a 63% chance because [] means you round down.