Playing is free but you can buy 'diamonds' in their website's shop which you can use as an in-game currency for all kinds of special rewards such as mounts, protecting your items from dropping on death, etc. They have a shop in-game where you can spend them.
Some of the items can also be bought using rubies (or whatever) which are obtained in-game, through daily quests I believe. You can also buy diamonds from other players, using in-game currency but they're very expensive. Gold is pretty much worthless later in the game, diamonds are used as the currency for expensive stuff.