Rebirth

When your Minions with Rebirth die, they leave behind a 0/1 Egg that hatches into the same Minion at the end of the next turn.

Rebirth is a specialty of Magmar.
An egg that is dispelled or leaves the battlefield has nothing inside of it and can never hatch. Dispelled eggs still count as eggs, though, and will work with cards such as Embryotic Insight.
Eggs hatch into brand new copies of the minion they contain. The new minion will have full Health, and won't have any of the additional effects it had before becoming an egg (such as Greater Fortitude). It will be able to act on the same turn it hatches.

It’s possible to use Egg Morph, Wild Inceptor or Morin-Khur to hatch your Eggs immediately.
Eggs can be buffed by your spells, and can sometimes make for a great surprise tactic!
It’s best to leave the killing blow of any Rebirth minion to your opponent, as they will not be able to stop the Egg from hatching if they don't kill it on the turn they cause the Egg to spawn.