The people (with faces) are the Operators, aka Tenno, and the faceless monster-cyborg-alien-ninja things are the Warframes. Without spoiling it too much, the Operators psionically remote-control the Warframes through special powers tied to an inscrutable warp dimension called the Void, and are also the power source for the Warframe's abilities beyond its raw strength and speed. A Warframe without an Operator is either inert/dead, or an uncontrollable human-eating monster, depending on which source you go by.



Exactly what the Warframes are, where they came from, what they're made of, etc. is not clear yet lore-wise, but a big quest is supposed to come out early next year which will reveal that.