Joke Groups are group names that are used for releasing things that established Release Groups don't want to be associated with. It's an alternative to packing releases as Independent.
There are several types of joke groups. Some are established groups that work as subgroups for releases the main group don't want under their own names (for example, see Rapid Fire  or Cezar ).
Some are one off jokes that only release or two things before being scrapped.
Others are a label that several people use as a funnier alternative to the plain Independent label. Sometimes these can take on a life of their own, see BatMan  as an example, and read their NFOs for a laugh.
Paracox  is another typical example of a joke group.