Spell Preparation

This rule applies to all character classes that prepare daily spells.
Instead of preparing each spell individually, a spellcaster prepares a list of spells that are available to cast. A character can prepare a number of spells according to the Spells per Day table for that class, including bonus spells. A character's Spells per Day also defines how many spells of each level can be cast that day. However, casting a spell does not remove it from the character's list of prepared spells. Instead, it only counts towards his limit of spells cast per day.
For example, a 3rd level wizard with an 18 Intelligence can prepare up to 4 cantrips (level 0), 3 level 1 spells, and 2 level 2 spells. He can cast one of his prepared 2nd level spells using either of his 2 slots for that level. This casting uses up a spell slot for the day, but does not remove the spell from his list. So the wizard can cast the same spell again later using his other 2nd level slot, or he can cast the other spell he has prepared, if he so chooses. If this wizard casts a 1st level spell, he can use one of his 3 slots for 1st level, or even use a higher level slot. Likewise, he can cast a cantrip using any of his spell slots.
In essence, a spellcaster that prepares spells casts his spells spontaneously from the list his has prepared for that day. However, this does not affect casting time for metamagic feats as it does for other classes that normally cast spontaneously, such as the sorcerer.