A sketchlangor sketch is a conlang with few details. A sketch is to conlangs like a stub is to wiki pages.
Many sketches never reach full maturity, and never receive a full grammar or vocabulary. Usually they are quickly put-together (or 'sketched'). However, carefully-constructed sketchlangs do exist, and nearly every conlang starts out as a sketch.
Another definition of a sketch is a conlang that has been abandoned or put on hiatus in early development.
Many naming languages never get beyond the point of being sketchlangs, and a large number of fictlangs are themselves sketchlangs.
For a list of sketchlangs which have a page on FrathWiki, see the Sketchlangs category.