The premise is quite simple. First, find a useful Wikipedia article that normal people might read. For example, the article called "Knight." Then, find a somehow similar article that is longer, but at the same time, useless to a very large fraction of the population. In this case, we'll go with "Jedi Knight." Open both of the links and compare the lengths of the two articles. Compare not only that, but how well concepts are explored, and the greater professionalism with which the longer article was likely created. Are you looking yet? Get a good, long look. Yeah. Yeeaaah, we know, but that is just the tip of the iceberg. (We're calling it Wikigroaning for a reason.) The next step is to find your own article pair and share it with your friends, who will usually look for their own pairs and you end up spending a good hour or two in a groaning arms race. The game ends after that, usually without any clear winners... but hey, it beats doing work.
A much longer list of entries to compare is at jwz's blog.