Roleplays, often abbreviated to RPs, are the trainings of Starfleet'. Sometimes you may have a training without doing a roleplay, in which case it's just a training. These can be canon, or non canon. If they are canon, they get put in the Mission Log article. If not, they are basically forgotten. To be canon, it must fit with the storyline, and any characters that die must stay dead.
Usually a RP starts when an MR or HR posts the shout "RP at (placenamehere)", this person is called a "host". When people show up, usually the person who posted the shout is in charge of the setup and everything else, so that's where things happen differently sometimes. Most of the time, the roleplay is acting out a fake episode of Star Trek, and it usually takes place mostly on the bridge. Also, the captain of the ship for that RP is usually the one who hosted it in the first place. Depending on who hosts, the host or the trainees get to decide most of what happens.
Trainings aren't roleplays, they are actually very different. Trainings are usually something to do with things like how to fly a shuttle, how to use phasers, discipline, etc. The same rules and restrictions for RPs apply to trainings.