Despite their position as the main enemy in Halo, we know very little about the Covenant. What we do know is that they are some sort of religious alliance between at least five, but probably six or more, species, and that they hate humans for some unknown religious reason. These races include, but may or may not be limited to, Grunts, Jackals, Elites, Hunters, the "engineers" seen by the Master Chief in The Fall of Reach, and the "priest caste" mentioned in the The Fall of Reach. It is unknown whether or not the Priests are an entirely separate species or just a higher level of another species.

The Covenant have a relatively high level of technology; most of their weapons are plasma-based, which means that whatever power sources the Covenant weapons have, they must be incredibly powerful. Their starships are much faster than human vessels, and are capable of pinpoint maneuvering while in FTL travel, as is made explicity clear several times in The Fall of Reach, when Covenant ships materialize right in the middle of a human battlegroup.

Halo, according to Capt. Keyes, holds some deep religious power over the Covenant. The ring is extremely important to them; so much so that they would abandon the trail of Earth to follow a single human starship headed in its general direction. They believe it to be a weapon; why they believe this, and how they intended to use it as a weapon, is unknown.

After the destruction of Halo, the Covenant juggernaut still looms large over Earth, and the task is likely to fall upon the Master Chief, and any remaining Spartans, to fight them back while simultaneously discovering the secrets of the other Halos and keeping the Covenant from using them, however they might do so, against Earth.