Capes are used for PvP (player Vs Player) fighting, it is just for fun at the moment as when you equip a cape you can kill or be killed by anyone else wearing a cape of a different colour, or if you wear the gold cape (i think thats the one) then you can be killed or kill anyone with a cape equipped.
Being killed doesnt loose any XP or SP, if you die you get the option to res where you are (on the same spot you are laid) or just lie there dead.
You dont collect capes for any reason however you can own more than one cape and also own more than one colour of cape.
You equip a cape like you equip a job item, so just pop it into the bottem right slot and away you go.
I use PvP to get to know people, it freindly fun normally and doesnt cost you any XP, it does however reduce your MP and HP so if you have auto pot turned on i would suggest turning it off when you fight and only manually healing if you really want to
Anyways, hope this helps
FFGIRL