Quote from: Cuer
Evasion Skill is what you see on your character screen, and is set at a capped level based on your Job. There are very few items that raise Evasion Skill; it is thought of as being a more powerful way to raise a character's evasion rate.

There is also an internal Evasion number (which is not displayed anywhere that players can see), which is based on other things such as a character's Agility. This Evasion is raised by a large number of different items, can be reduced by Gravity, etc.

Nobody knows what the exact calculation is for Evasion (well, except Square-Enix, and they aren't telling), but obviously having more of either one causes opponents to miss hits on a character. Tanking Ninja tend to want to load up on +Evasion items, since it helps boost their already high evasion rate.