RPs... *GRowls* Fine, I'll give him an explanation.
RPs come in hugely massive different forms. You can play them over forums, over the internet, on pen & paper, on games on WHATEVER THE HELL YOU WANT! Because RP is extremely flexible.
The Dark Heresy RP is a (Well, technically) Pen & Paper RP, where you create a character sheet and through various rolls and tests attempt to lead him/her through the adventure as best you can. For example on our Dark Heresy, we're using an internet based rolling system to decide what happens in combat etc..
However, this is not the only form of RP. The more popular and more widely played form is, in simple terms, creating a story with alot of different people. Each person has 1 character or more, and generally one person is the sort of "Leader" of the group, though this doesn't always happen (It's more specific to Forum/Pen & Paper RPs, whereas most Game based RPs don't have a leader). The "Leader" plays like a normal character, however he/she will also make alot of key story/plot based posts (See Hic In Suus Nomen). In RPs without a leader, everyone contributes to the story and makes it up as they go along, being it characters dying or being attacked.. etc... etc...
However, they're also a large amount of rules posted here:
RP Rules
But the most important rules are:
Don't God-Mod yourself. This means don't make your character stronger and better than everyone else, and automatically kill everything. This is why we disallowed Space Marines.
And - Don't try and control other people's characters. They know what they want to do with them, where they want to go, and indeed what their character's personality is like. It really doesn't work well if someone else tries to control your character for you. They'll end up doing/being something you don't want them to.
Thanks
TheKiller