A funlang is a 'just for fun' conlang. Of course, many conlangs - including many famous and ambitious ones - are made just for fun. In this sense, Quenya and Sindarin could be considered funlangs.
More specifically, however, a funlang is a small conlang project done without much afterthought or high aspiration, as a distraction from more 'serious' conlang projects. A funlang in this sense usually has no detailed conculture or conhistory attached, the grammar is and will remain sketchy, the vocabulary is small, etc. It is done without much effort. An example of a funlang is Gobldi Guk, as opposed to the more ambitious Albic project of the same author.
Many funlangs are also sketchlangs due to their nature.
A subset of funlangs are jokelangs, which are conlangs created exclusively for the purpose of amusing other people. A good example of a jokelang versus a funlang is Auld Elvish, a parody of Tolkien's Sindarin and Quenya. Another jokelang is DiLingo.
Funlangs on Frath
A number of funlangs have a hosted description on Frath. To see a (mostly complete) list of the avilable ones, please click here.