this page is outdated - look here : Cleric Domains
Clerics specialize in two areas of magical expertise called domains. These domains are selected when first becoming a cleric and cannot be changed. Domains grant added spells and sometimes a free feat or special ability.
A cleric gets one bonus domain spell slot at each level they receive a domain spell. When a cleric prepares a spell in a domain spell slot, it should come from one of his two domains (but NWN2 does not enforce this).
A cleric chooses two domains at their first class level, ideally representing the deity they have chosen to follow. Each domain gives the cleric access to at least 2 domain spells he can cast that is added to his spell list, as well as possibly gaining a granted power. The cleric gets the granted powers of both the domains selected.