From Halopedia, the Halo wiki
Humans are a mammalian species that expanded out into the galaxy. After a disastrous first contact, humans are facing near extinction due to the Human-Covenant War. The UNSC, the primary fighting force for the human species has attempted to fight for survival but has now been pushed back to their cradleworld - Earth. John-117, one of the most legendary SPARTAN-IIs is Earth's and humanity's best hope for survival.
The Elites are a saurian species from Sanghelios, who were one of the founding members of the Covenant and one of the former leadership castes. The Hierarch Prophet of Truth betrayed the Elites beginning a large scale civil war. Some Elites have joined the separatist Fleet of Retribution led by the Arbiter and have allied with humanity.
The Brutes are mammalian species from Doisac, added relatively recently to the Covenant. With the help of the Prophet of Truth, the Brutes have replaced and become embroiled in a civil war with the Elites. The Brutes remain steadfast and loyal to the Prophet of Truth. Their most fearsome warriors are the Brute Chieftains who wield the Brute created gravity hammer.
The Grunts are a small, methane breathing arthopodal species from Balaho. Acting as the frontline infantry of the Covenant for entire war, a number of Grunts have chosen to side with the Brutes and the Prophet of Truth. Unlike other members of the Covenant, they breathe through specially designed methane tanks on their backs.
The Jackals are an avian species from Eayn. Mostly known for being pirates, the Jackals have maintained a position in the Covenant for hundreds of years. Following the beginning of the civil war, they have chosen to side with the Brutes and Prophet of Truth. When not in sniping roles, they are often seen with their wrist mounted shields.
The Hunters are a formed colony of small worm-like creatures named the Lekgolo from the gas giant of Te. One of the more enigmatic species of the Covenant, some Hunters have chosen to side with the Brutes and Prophet of Truth where they act as a large, formidable force.
The Drones, or Buggers, are a flight-capable insectoid species from Palamok. Like the Jackals, the Drones have been a part of the Covenant for hundreds of years. As with other species, when the civil war broke out, many Drones chose to side with the Brutes and the Prophet of Truth.
The Flood is a hostile all-consuming parasite that nearly destroyed all life in the Milk Way 100,000 years ago. The Flood that was being studied on Delta Halo, managed to escape the ring thanks to the battles going on there and infect the mobile station/city of High Charity. Now, the Flood is positioning itself to infect Earth and wipe out all the species of galaxy.