Crysta is Square Enix's virtual currency that is kind of like Xbox Live points that you can use as a way of paying for FFXIV's (and FFXI's) subscription. You can't actually use it to buy in game items at the moment though, only subscription fees. You can use various payment methods to 'buy' Crysta in any amount you wish, and then use your Crysta balance to purchase subscription time for the game.