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 domain must be supported by a Cleric's Deity.
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.
Each domain gives the cleric access to a domain spell at each spell level he can cast, from 1st on up, as well as a granted power. The cleric gets the granted powers of both the domains selected.
With access to two domain spells at a given spell level, a cleric prepares one or the other each day in his domain spell slot. If a domain spell is not on the cleric spell list, a cleric can prepare it only in his domain spell slot.