From D&D Wiki
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Each skill point you spend on a class skill gets you 1 rank in that skill. Class skills are the skills found on your character’s class skill list. Each skill point you spend on a cross-class skill gets your character 1/2 rank in that skill. Cross-class skills are skills not found on your character’s class skill list. (Half ranks do not improve your skill check, but two 1/2 ranks make 1 rank.) You can’t save skill points to spend later.
To make a skill check, roll: 1d20 + skill modifier (Skill modifier=skill rank + ability modifier + miscellaneous modifiers)
This roll works just like an attack roll or a saving throw— the higher the roll, the better. Either you’re trying to match or exceed a certain Difficulty Class (DC), or you’re trying to beat another character’s check result.
Epic Skill Uses
Despite the near-magical nature of some epic level skill uses, all uses of skills are considered exceptional abilities (except if noted otherwise), and thus function normally even within areas of antimagic.
A character’s number of ranks in a skill is based on how many skill points a character has invested in a skill. Many skills can be used even if the character has no ranks in them; doing this is called making an untrained skill check.
Miscellaneous modifiers include racial bonuses, armor check penalties, and bonuses provided by feats, among others.