Domain was originally introduced in Invasion block, and the mechanic had the nickname of domain but did not officially have a keyword associated with it. This is because when originally introduced ability words did not exist, they were only used for specific abilities with associated rules in the comprehensive rules. Card that with a similar mechanical feel but different rules text did not have specific official names.

When Conflux was released in the middle of Shards of Alara block domain came back and was given a an ability word. When that happened, previous cards from Invasion block had their Oracle text changed so they all now officially have the ability word of ‘domain’ on them.

So, with that out of the way, how does domain work. Domain abilities have variable effects based on the number of different basic land types you control. For example, Matca Rioters has a power and toughness that depends on the number of basic land types you control. If you control a swamp, forest and mountain then the Rioters will be a 3/3 creature.

Domain abilities do not care about the number of lands you control, or the number of lands with a specific land type. Domain only counts the number of basic land types you control. So, if you only control five islands, you will only look at the top one card of your library when resolving Worldly Counsel. This also means that lands with more than one basic land type will be counted twice when calculating domain effects. For example, a lone Sacred Foundry will count as both a mountain and a plains.

Some domain abilities generate continuous effects, like Matca Rioters above or Manaforce Mace. These effects will continually check what basic land types you control to determine the effect. Other domain abilities create effects when a spell or ability resolves, such as Drag Down. The check for the number of basic land types you control only happens once, when the spell resolves.