The main ingame currency is Astral Diamonds (AD). Zen is the cash shop currency and can be bought with real word money (obviously) and can be exchanged for Astral Diamonds, based on a the economy's exchange rate. With Zen you can purchase XP boosters, extra storage space and bags, life scrolls, mounts, companions, etc. The real carrot, though, are the keys that open the purple chests you find, that can only be purchased with Zen.