See also: Spellcasting.
Certain spells have a special tag: ritual. Such a spell can be cast following the normal rules for spellcasting, or the spell can be cast as a ritual. The ritual version of a spell takes 10 minutes longer to cast than normal. It also doesn't expend spell points, which means the ritual version of a spell can't be cast at a higher level.
To cast a spell as a ritual, a spellcaster must have the ritual casting feat. The caster must also have the spell prepared or on his or her list of spells known.