The idea of these is sites is to post a comment/move in whatever games take your fancy, then come back later to see what others have added. The pace is sedate - you get a few new moves on each site each day. It's a bit like Twitter in that respect, I suppose.
As far as how to play individual games, well that varies. The simplest are games like the limerick or haiku games, where you just post the next line to a limerick or haiku (OK, they're senryu really, usually with a comic tone). In most other games we're just making things up as we go along.
Are you familiar with I'm Sorry I Haven't a Clue? This site is a web-based version of this long-running comedy radio show. We try to play most of the games they play on ISIHAC, and invent our own in the same style. That's what this site – and Orange and MCiOS contain.