What's exactly is Zen and why would I need it?

Okay, I'm still downloading/installing the game, but I'm reading everything I can find on the main site while this is going on.

My biggest question is: What is the point of this Zen stuff and is it only available as a real-world purchase, or is it available in-game for a certain amount of gold coins?

If I decide that I like the game enough to keep playing, I should be able to refer enough other people to qualify for the Black Raptor mount, but if you have to pay real money for Zen, none of them will go for it.

- Treylan