
Square Enix has revealed the method of character growth in Final Fantasy XIII. Instead of a traditional level-up system, FFXIII will introduce the Crystalium System.
Those familiar with the Sphere Grid from Final Fantasy X won’t have too much trouble adapting to this new system. In Final Fantasy XIII, characters will earn Crystalium Points, or CP, from battles — instead of the traditional Experience Points. The CP can then be used to unlock nodes on a grid, which grant character traits and abilities. Unlocking a node allows you to progress your abilities further down that area of the grid, enabling abilities beyond that node to be unlocked.

The Crystalium System places character growth on an expanding grid, allowing earned CP
to unlock nodes and create a path to desired traits, magic and abilities.
Magic upgrades will work in the same way, allowing the familiar progression through the tiers of Final Fantasy spells, such as from Fire, to Fira, to Firaga.
Presumably, players will have to choose which abilities they want their characters to unlock to make them most useful in battle.
Final Fantasy XIII will be out in Japan on December 17, with the English translation due to hit North America and Europe in Spring or early Summer of 2010.