Incarnation
Incarnation literally means embodied in flesh or taking on flesh. It refers to the conception and birth of a sentient creature which is the material manifestation of a force or concept whose original nature is immaterial.
As a creature type it first appeared in Judgement. Personal Incarnation from Alpha gained the subtype Incarnation retroactively.
Judgement cycle
Each of these creatures has a static ability that grants all of your creatures an in-color ability (haste for red, flying for blue, ...) as long as it is in your graveyard and you control the relevant basic land.
Genesis and Glory are incarnations with similar abilities in that their abilities only function while they are in the graveyard, but are not part of the cycle.
Lorwyn cycle
Inspired by the original cycle, Lorwyn had a cycle of rare Elemental Incarnations which grant their controllers a powerful effect. In direct contrast to the Judgement cycle, however, the Lorwyn incarnations each have a triggered ability that shuffles them back into their owner's library should they enter the graveyard.