Domain — At the beginning of each player's upkeep, that player gains 1 life for each basic land type among lands they control. Then this enchantment deals 3 damage to that player.
Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.
The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.