Now we've got that out of the way you might want to consider going to the RS site and reading up on it there, cause it has surprisingly large amounts of information in a real convenient form there.
Nope, didn't think you'd be too happy to that idea so here's a quick run down.
Its for all players, member and non-member.
Its a place where you can try and sell your items or try and buy items on a single massive marketplace.
It in Varrock.
If your selling you put your items in, say what price you want to sell them at (within certain boundaries set by Jagex. NO, get that idea of an incredible RWT system out of your mind right now! ITS NOT GOING TO WORK), and then wait for someone to buy them (after which point you get your money).
If you want to buying then you select how many items of what sort you want to buy and at what price (within certain boundaires set by jagex. if the voice in your head says RWT SCAMZ then turn the fucking stereo up higher!) you want to buy them at and then wait for someone to put them on the market.
Anymore questions then RTFM!!