Play-by-post is exactly what it sounds like - an RP that runs a post at a time. This can range from simply a cooperative story, to a standard tabletop ruleset, just being run a post at a time.
Since each thread -is- the RP in question, anyone creating one should of course state the setting, rules, etc, in the initial post. If you're doing something more complex - say, a world where each thread is a play-by-post RP, but there's an overarching setting and characters, feel free to request a subforum to run that out of.
Anyone is free to run either type of RP here, as well as to discuss the medium, ask for help making characters, etc.