Gods are divided into two types: elder and younger. Elder gods are born randomly from chaos nebulae and have the power to create entire universes inside their minds. Younger gods are, generally, a being with godlike powers born or created inside these universes. They can be the children of elder gods, an aspect of the creator's personality that gains sentience, or another creation or phenomenon with sufficient power or responsibility. A mage with the right skillset could become a younger god, for example.
There is a third type, but only one of this type has ever been recorded: chaos gods. This theoretically happens when an entire chaos nebulae gains consciousness.