There are usually four altars in the game: an Altar of Water recruits water elementals, an Altar of Fire recruits fire elementals, an Altar of Earth recruits earth elementals, and an Altar of Air recruits air elementals. The names of the altars varies from game to game - for example, in the Price of Loyalty, the Altar of Air is called the Air Summoning Altar. In Heroes V, the elemental do not have seperate altars, but can all be found at the Elemental Conflux.
Elemental altars can be found on the adventure map, and in Heroes III: Armageddon's Blade and Heroes IV, they can also be found in towns - the Conflux town and the Preserve town, respectively. The Conflux includes an additional altar, the Altar of Thought, which trains psychic elementals. The Preserve altars (known as Portals) cannot be built, but they can be used when a creature portal has been built.