The Caretakers - as they're referred to by at least some Humans - are a hissing insectoid species with a basically humanoid body plan (two arms and two legs) which build nests to live in. They are native to a desert world orbiting two suns, which might be located in a different dimension, as it's linked to a Human world (possibly Earth, though that's not confirmed) through some kind of portal. They're seemingly hostile, given that the Second Doctor fought against them.