Factions

There are currently four distinct Factions in Warframe, along with two others which are less documented. While it is never outright stated, every known faction appears to be (or have once been) human.

The only playable faction are the Tenno.

Other factions such as Grineer, Corpus, Infested and Orokin are presented as adversaries.

Tenno
"The Tenno are descendants of an ancient and mystical civilization of lost warriors from the Orokin era on Earth. Preserved in cryopods for centuries, the Tenno now awaken to a new war, fighting and resisting warring factions as the sole bearers of the Orokin-created Warframes.  While the memories of the Tenno have faded over time, their mastery of guns, blades, and Warframe exo-armor has not. Fragments of history suggest that discipline and chivalry are the cornerstone values of the Tenno: is this true today? The Tenno are emerging into a world unfamiliar to them. One sees a noble warrior, building his strength against an oppressive regime. Another sees an opportunistic mercenary, exploiting the Warframe's superiority for wealth. Regardless of their future, the Tenno stand united against a common foe, loyal only to each other."

- Warframe Official Website

Grineer
"Vast in numbers, the Grineer have expanded their vile empire from Earth outward to all sectors of the solar system. Once the Grineer rediscovered cloning technology, they seized control of Earth through sheer numbers. While they may be at the height of power, the Grineer army is in bodily decay and their features are increasingly primitive as a result of continuous cloning. As such, the Grineer are forced to use technology as a crutch for their genetic deterioration. An ideology of hate and discrimination towards the Tenno continues to drive the Grineer forward in the face of inevitable dissolution."

- Warframe Official Website

Corpus
"The Corpus are a dominating proto-corporation of humans that maintain and operate the trade routes of the Solar Rails. They are rarely seen by others and are better known for the robot proxies that serve for them. The social structure of the Corpus is said to reflect ancient mercantile guilds from the Old Earth. Their production methods blend technology and ritual, in-part, because the ancient Orokin materials they use are inscrutable to them. They control the entire supply chain of goods that flow through the solar system. The Corpus view Orokin artifacts as resource to be exploited for profit− at any cost."

- Warframe Official Website

Infested
The Infested are what appear to be humans who have been taken over by some sort of virus or parasite. The majority of Infested seem to be former Corpus Crewmen or Grineer Lancers, however they are also speculated to originate from failed Tenno experiments, hinted at by J3-Golem asking the Tenno, "Why do you destroy us? We are your flesh," yet, have also been hinted at by several references to a "Technocyte Virus" in some missions. Most Infested do not have any significant ranged combat ability.

Orokin
The Orokin are a precursor race that ruled over the Solar system for a period of time. During what was known as the Orokin Era, they built technological masterpieces that rival even present day technology. During a war with what were known as the "Sentients" the Orokin created the Technocyte Virus as a way to combat them. Very little information remains on what their goals may have been.

Stalker
These are random encounters and are usually high level (30-70). The Stalker possesses preexisting abilities from other Warframes: Excalibur's Slash Dash, Ash's Shuriken and Teleport. It is capable of taking out low-leveled players without much trouble and can even prove troublesome for groups, though it usually only targets one player until that player is dead, and then departs. There have been occasional encounters with level 1 Stalkers, perhaps due to host lag or glitches.