[Draggor]

In full 3rd ed rules a cleric gets to choose two domains out of the selection for his diety. Each domain gives you some sort of bonus, either a new feat, some special ability or bonuses to certain rolls. Each domain also comes with a list of spells for levels 0 to 9. A cleric has to memorise at least 1 of his domain spells for each level of spells he can cast. So in most cases a cleric will get to choose 1 of 2 possible domain spells to pray for.
In never Winter nights domains simply give you access to spells earlier or spells you normally couldn't cast. They have been made simpler and you don't have to memorise a domain spell(?).