The Forerunners, as one might deduce from the name, are an early race of beings in HaloMarathonHalo. Little is known about them except that they apparently built Halo, Halo's Monitor (343 Guilty Spark), and the Sentinels. They no longer exist, but we can assume that they were approximately human-sized, because all of the buildings on Halo were scaled to human proportions.

We also know they created the crystals that the Covenant are after and that these crystals make it possible to travel through time. Which I find quite strange is that some organic lifeform creates a weapon capable of destroying all organic life in the universe, why would they do something like that? That weapon would destroy them as well.