The Ruach are a group of people that predate current residents of Temerant. They appear to have longer lifespans than the people who reside in Temerant during Kvothe's time, with notable members being Selitos, Lanre, Iax, Tehlu, Kirel, Deah, Enlas, Fair Geisa, Lecelte, Ordal, and Andan. 
The ruach are described by Selitos as pure of heart. They are most likely humanoid, but based on Felurian's assertion that none of the original Amyr were mortal, we can assume they are immortal.
According to Skarpi's story, after deciding to follow Tehlu, 8 of the ruach (Imet, Kirel, Deah, Enlas, Geisa, Lecelte, Ordal, and Andan) had their names spoken by Aleph and became beings of fire, wreathed in flames which consumed them so that they disappeared from the sight of men.
Most of the Ruach hung back from Selitos, too. They were afraid, and they did not wish to become involved in great matters.
Ruach may be an earned title, and all human residents of Temerant may not have been Ruach, or Ruach could have been a general name for all people at that time.