The Overseers for Earth are genetically modified humans with some nanite prosthetic devices. The Overseers are few in number, but they are in control of Observer Base on the Moon. The Overseers enforce the Rules of Observation, rules that prevent Earthlings from become aware that Earth is being observed.
Overseers have been genetically engineered (they have great physical strength and a long life span) and are no longer very similar to Earthlings. They make use of nanites to erase the memories of Earthlings who become aware of the fact that Earth is being observed. Using nanite prosthetic devices, Overseers can change their appearance and their voice.
Normally there are no Overseers on Earth, but they go to Earth when needed, sometimes in order to remove Earthling from their home world, particularly when they become involved in an Interventionist plot.
Overseers are trained to believe that they have technology (space craft and nanites) that is superior to the technology available to Interventionists. This is not always the case, and there is actually a dynamic balance of power between the Interventionists and the Overseers. The Huaoshy make sure that the balance of power between Overseers and Interventionists is maintained. The Huaoshy can then either slow or accelerate the pace of cultural/technological development on Earth as is appropriate.