It's in game currency. I bought the game years ago, and have yet to spend any more money on it. The developers felt bad that at launch years and years ago, the game was so unfinished that it was called a scam. So they've been uploading free content, and updates ever since. At this point, I'm fairly sure most people would agree that they've lived up to the original promises and then some, but they still keep cranking out updates. Hey, I'm not complaining.

I would say more specific things, but I don't want to spoil it. Playing the game is the experience, and the experience IS the game. If you come into this game thinking "I'M GOING TO MAKE ALL THE MONEY AND GET ALL THINGS!!!" then you're missing the point of the game. The journey is the experience. Think of it like getting a bunch of random assorted legos, rather than a lego kit. The fun is in the creativity of what you do with it.