Altars are special buildings that begin to unlock at level 20. The boost your heroes' stats permanently after being built. Once they are placed, they start working immediately. For example, an Earth Health Altar increases all your Earth heroes health. By default they increase the stat by 2%. But they can be upgraded to increase this stat boost.
Element Stones are special stones that are used to upgrade Altar buildings. There's a building for Attack, Defense and Health boost for every element.