Yerf is cartoon-like art that represents creatures that have features of both human beings and of animals. Yerf is also referred to as anthromorphic or furry art (see Anthropomorphism). It is a phonetic reversal of the the word "furry".
[Bugs Bunny]? is the archetypal furry. He clearly isn't a rabbit; he walks on two legs and has hands. Yet he clearly isn't a human; he's yerf. Other examples include the race of humanoid ducks found in the role-playing game RuneQuest and the races found in the Sonic the Hedgehog animated cartoon series.
There are a large number amature yerf sites on the Internet.
Mature, adult, or pornographic yerf is also known as yiff? or spooge? (note, however, that "spooge" is a generic term that is used to refer to all sorts of highly explicit and gratuitous pornography).
See also, Ernest Shepard