When a unit receives 100 experience points, they level up. A random assortment of stats increase when they level up. Each stat has a unique number per character called a growth rate. These rates are taken out of 100 and are the percent chance that the stat will go up by 1. Few characters have growth rates over 100; with luck, these characters may receive 2 points per level.
Most units stop leveling when they reach level 20. However, most units in FE9 and FE10 can reach level 21, where they will automatically promote to the next class. Most can also be promoted at level 10 and up. Laguz stop leveling when they reach level 40 in Radiant Dawn.