The Forerunners are an ancient alien race in Halo Wars that has left precious artifacts around that can help other factions as they war with each other. They are considered a neutral faction in the game, and may not actually appear as living lifeforms, but only as their artifacts that can be used for assistance.Halo Wiki: http://halo.wikia.com/wiki/Halo_Wars#Forerunner Halo Wars Forerunner]
The Other Factions
In addition to the ancient Forerunner race, there are three other factions in Halo Wars. The other "neutral" faction is the Flood. The Flood are still very much alive after their war with the Forerunners, and they now feed off of other lifeforms in the galaxy. The Covenant calls them "The Parasite."
The two major factions that are playable in the game are the Covenant and the human race known as the UNSC. The UNSC are who players will control during the campaign mode, but the Covenant are also playable in the multiplayer modes.Halo Wiki: http://halo.wikia.com/wiki/Halo_Wars#Forerunner Halo Wars Forerunner]