A pong is an often comical and/or embarrassing, written IRC post or paragraph that has an obvious, non-intended, Freudian slip. This term originated on the Yiffnet #furry channel, and they were so popular, both innuendo and mirth, that Tet Solfire started collecting the best in Tet's Pong Database. A wem is, simply, an intentional and/or fake Pong, used to attract attention to one's self, named after its originator.
The tradition survived the Yiffnet's split, to continue on FurNet's own #furry channel. But with the move to Anthrochat of the majority of FurNet's #furry denizens, the term is now hardly used, if at all.