@Ionus,
Well, the doctrine is that after this life, one will be re-born in one of six realms in the next life. These are Heaven worlds, Asura worlds (asuras are like demi-gods), human realm, animal realm, hungry ghost realm, hell realm(s). The duration of your existence is determined by your karma. In the Hell and Heaven realms, the lengths of time are depicted as pretty astronomical - but not eternal. There is nothing eternal, as in 'of limitless duration'. Of course, the Buddhas, Arhats and Never-returners are completely 'out of the loop' by definition. The Bodhisattvas incarnate in various realms 'for the welfare of all sentient beings'.
The best of all possible births, if one is to be born at all, is in the human realm, as only in this realm is it possible to hear and comprehend the Teaching. Those who fall back into the lower realms may be consigned to them for 'aeons and aeons of kalpas'.
That anyway is a thumbnail sketch of the traditional doctrine.