From Halopedia, the Halo wiki
The United Nations Space Command (UNSC) is the military arm of the Unified Earth Government, the primary governing body of humanity's interstellar empire. Although it had much experience countering rebellions and insurrections throughout its history, the UNSC's mettle was first truly tested by its war with the Covenant, a conglomeration of aliens who viewed humanity as an affront to their gods and sought to exterminate them. The UNSC has fought a losing war against the Covenant for twenty-eight years and now has its back against the wall defending the human homeworld - Earth.
The Covenant separatists are the former-Covenant personnel led by the Arbiter now caught up in the civil war against the Prophet of Truth. Primarily composed of Elites, the separatists have allied themselves with the UNSC in recognition of their mutual enemies - and may be the only guarantee for humanity's survival in the near-future. The separatist Fleet of Retribution is one of the most visible elements of the separatists' military power.
The Covenant is a union of several alien species, brought together by faith and fervor to worship the Forerunners and search for the Halo rings. Due to misinterpretations of Forerunner scripture, the Covenant believe that lighting the rings will allow all true believers to ascend into godhood, and will stop at nothing to achieve this. More recently, the Covenant has become embroiled in a bitter civil war between the former-leadership caste of the Elites and their instilled replacements, the Brutes. Led by the High Prophet of Truth, the Brute-led loyalists now pose a grave threat to all life in the galaxy.
The Ecumene was the ruling government of the Forerunners, before their civilization mysteriously vanished one hundred millennia ago. The Forerunners constructed the Halo Array and a number of other artifacts now scattered across the galaxy, and ultimately came into contact with the Flood. The Forerunners warred against the Flood for centuries, but were ultimately forced to use the Halos to wipe out all life in the galaxy to stop the Flood, and now their only relics are the gargantuan megastructures they left behind.
The Flood is an alien parasitic life form and the ultimate threat to life in the Milky Way galaxy. The Flood has been contained on Forerunner installations for millennia, and was able to escape thanks to the actions of the Covenant on Delta Halo. In control of the Covenant space station High Charity, the Flood is now poised to begin an invasion of Earth and finish the work they started 100,000 years ago.