Warframe is a free-to-play sci-fi cooperative third-person shooter. Players can choose between three factions (Tenno, Grineer, and Corpus) and then between 14 (at the moment) "Warframes" (i.e. classes). The story of the game is as follows:
They were called Tenno. Warriors of blade and gun: masters of the Warframe armor. Those that survived the old war were left drifting among the ruins. Now they are needed once more.
The Grineer, with their vast armies, are spreading throughout the solar system. A call echoes across the stars summoning the Tenno to an ancient place. They summon you.
Allow the Lotus to guide you. She has rescued you from your cryostasis chamber and given you a chance to survive. The Grineer will find you; you must be prepared. The Lotus will teach you the ways of the Warframes and the secrets to unlocking their powers.
Come Tenno, you must join the war.