This is known as a freeform play-by-post (pbp) role playing game. Freeform because the rules are very flexible and allow for all the players to get creative with their writing without worrying about breaking them.
Playing is done by writing your character's actions in the forum that describes where your character is and responding to what other characters are doing. You can only respond to people's actions and play your character. You can't write for other people or control their characters (that's called cheating, or autoing) and you can't kill them unless they agree or a staff member OK's it.
Combat is not the primary goal of writing here, and so this section is quite short. If your character is in a fight, it is in your best interest to do a few things:
If you are comfortable with the people you are writing with, you may wish to come to an agreement with them to allow auto-hits. This means that "you can hit me if I can hit you" and can make everything much easier. Do this only if you're mature enough to be able to compromise for good writing and only if you have gotten to know the other person. If it causes a problem: take your losses, call it off, and don't cause a scene.
Remember: You cannot kill another player's character without having permission to do it, or unless a staff member says you can. That means that your play should not revolve around trying to kill other characters. Boo hoo. Make friends with people and work out a plot.
If a player stops responding to your posts and has not informed you that he/she is going to be absent for a while, you are allowed to skip their turn after you wait a courtesy period of 3 days. After this time, you can starts skipping them and carry on. If their timing is really bad and you cannot write something without them, you may ask a staff member for permission to write them out of your plot somehow. This can mean writing a few actions for their character if necessary.
Each of your characters can only be active in one place at a time. You are not allowed to join multiple threads at the same time in order to prevent confusion and to maintain plot continuity. An exception to this rule can be made for writing flashbacks ( external analepses ) that elaborate on past events involving your character. Any flashbacks must be clearly labeled as such.