Ability score

An ability score is the numeric rating of one of the six character abilities. Some creatures lack certain ability scores; others cannot be rated in particular abilities. (See also Attributes .)
The value of a character's ability score also provides a number of bonuses for spellcasting, attack rolls and other things.
==Increasing ability scores==
All characters gain a permanent +1 increase to a chosen ability score at 4th level and every 4 levels thereafter (4, 8, 12, etc.). At level 20, the character will have gained +5 ability points this way.
This increase can be used on any of a character's ability scores and is not restricted based on that character's previous choices.
For example, a rogue increases his dexterity from 16 to 17 at 4th level. When he reaches level 8, he can further increase his dexterity to 18 or may choose to increase any of the other 5 abilities instead.
In addition to this progression, characters may also gain ability score increases from other sources such as class features (e.g. barbarian rage) or spells such as bull's strength).