Halo is a science-fiction franchise based on a series of FPS video games, developed by Bungie for Microsoft's Xbox game consoles. While the games are the main canon, the franchise has a sizable expanded universe, with novels, comic book series, an anime series, and a live action miniseries. The games are incredibly popular, making the franchise very well known within the gaming community and in the general public.
The story takes place in the 26th century AD. Humanity, governed by the United Earth Government (UEG) and protected by the United Nations Space Command (UNSC), has been expanding through the galaxy with use of FTL technology called slipspace drives. This expansion comes to a grinding halt when they encounter the Covenant, an alliance of alien races bound together by strong religious beliefs. For mysterious reasons, the Covenant begin a genocidal war on humanity. The Covenant believes that activating the massive Halo rings left behind by the now-extinct Forerunners will let them follow the Forerunners to a higher plane of existence.
In truth, the Halo rings only serve to wipe out all intelligent life in the galaxy. The Forerunners used them in a last-ditch attempt to stop the Flood, a parasitic species that reproduces by taking control of other sapient life. Both the Forerunners and the Flood were wiped out by the Halo rings, though some Flood survived in a containment area on one of the Halo rings.
Into this situation comes the player, Master Chief John-117, a result of the Spartan supersoldier program. When an accident on a Halo ring causes the Flood to be released again, it falls to the Chief to go on near-suicidal missions to defeat both the Covenant and the Flood.
Unlike most video games, the abilities of the weapons and technology in Halo are somewhat well-defined because of its expanded universe, though a plethora of different writers does lead to inconsistencies. The UNSC and the Covenant are generally considered low-tier, somewhat below the UFP, though it is known that a Covenant fleet is fully capable of planetary sterilization. The Forerunners at their prime and the Flood when adapted to facing the Forerunners had a very high technology level, exceeding that of the Galactic Empire and approaching Culture level. Halo is often pitted against Mass Effect in vs. threads, likely owing to the aesthetic similarities between the universes.