e.g. A multiclass character with both cleric and sorcerer spells may only cast cure light wounds as a cleric spell, using a cleric spell slot, as cure light wounds is not part of a sorcerer's available spells. Similarly, the character may not cast the spell magic missile as a cleric spell in order to avoid arcane spell failure since it is not part of a cleric's available spells.
Sorcerers, bards, favored souls, and spirit shamans have a limited set of spells known derived from their class level. This is unaffected by ability score and won't increase from a high score. At level-up, additional new spells are chosen, while currently known spells may also be swapped from new ones.
A wizard begins play with a spellbook containing all 0-level wizard spells (except those from her prohibited spell schools) plus three chosen 1st-level spells. For each point of intelligence bonus the wizard has, the spellbook holds one additional 1st-level spell of your choice. At each new wizard level, she gains two new spells of any spell level or levels that she can cast (based on her new wizard level) for her spellbook. At any time, a wizard can also add spells found in other wizards’ spellbooks to her own.