Domains can be described as focus for clerics. What they do is help also create a broader feel to the class.
Examples of some domains
Air/Earth/Fire/Ice Domain - Use Turn Undead to Turn Earth/Air/Ice/Fire creatures like undead.
Chaos/Law/Good Domains - Cast the chosen alignment spell as +1 Caster
Death Domain - Touch spell 1d6 per caster level if the amount is greater then the HD of the creature it dies (Might need a good cooldown on this one)
Healing Domain - +1 Caster level to healing spells
Luck Domain - Reroll a failed roll
Magic Domain - Use of Scrolls/wand and other devices as a Wizard of 1/2 the clerics level
See
http://www.d20srd.org/srd/spellLists/clericDomains.htm
All of these have specific spells granted to memory (not trainable they just have them to cast)