Yokians have slimy green skin, which turns white to exposure of heat. They have a lower percentage of water inside their bodies, and they don't socialize with carbon life forms. The race is so advanced that they have found no need for their original bodies; instead, they rely on egg-like structures to hold their bodies in.
In Jimmy Neutron: Boy Genius, Jimmy said that the aliens were advanced species, since they had no need for their original bodies. Sheen said in reply that his boogers look like an "advanced species", to which Cindy and Libby were grossed out.
The Yokians live in monarchy under the reign of King Goobot. They've tried for countless millennia to find sufficient sacrifices for their god Poultra. A couple minor, normal Yokians made cameo appearances in "Win, Lose and Kaboom" and "Jimmy Timmy Power Hour 3: The Jerkinators!".