Tenno
The independent warrior community formed when the Zariman children bonded with Warframe bodies through Transference.
ZARIMAN · VOIDFULL SPOILERS
The child and the war body
The Operator or Drifter carries consciousness and Void power; the Warframe carries the biological body built for war. Transference joins the two in the experience of a second body, and the word Tenno usually covers that combined warrior identity as well.
The revolt against the Orokin
After the Sentient victory the Tenno destroyed the Orokin leadership. Natah's plan matters, but so do Margulis's death, the Warframe experiments and the empire's cruelty as historical causes of that strike.
Their role today
They found no state of their own. They aid the Ostron, the Solaris, the Holdfasts and other communities, and keep either the Grineer or the Corpus from ruling alone.