Clerics and Druids can use spontaneous casting to cast spells they haven't prepared in exchange for losing a spell they have prepared.
Sorcerers, bards, favored souls, and spirit shamans cast their spells without having to prepare them in advance. They may cast any of their spells known, however they generally know fewer spells than preparation spellcasters.
Clerics recieve a bonus spell slot for each spell level from 1st level upwards. This slot may not only be used for the preparation of spells gained from the cleric's domains, but for any spell she knows.