You get bells for making posts, that sort of thing. It's small amounts though - you can check how you've earned your bells by going to your profile, and then on the top bar with white links the last one says "Currency". When you click on "Bells" there's a link that says transactions and it will bring you to a page like this: http://www.belltreeforums.com/credits.php?u=50812¤cyid=1
You can also earn bells, by selling stuff. In-game items, mostly unorderable items and DLC sell over at the TBT Marketplace. And, as Bunnee said, you can sell in-game-bells (or IGB) for bell-tree-bells (BTB), and vice versa. If you're an artist you can also earn bells by selling art commissions down at the Museum.
TBT (also BTB, they're the same thing) is the forum currency. It's like an incentive to get you to be active on the forums. I guess I should have mentioned that! I didn't really get it either, until I realized that they're actually useful if you want to get rare items for little in-game work.
The only places you DON'T earn BTB by posting is in the Basement (which is the part of the forum where you play games), and I can't remember if you earn BTB in Brewster's Cafe.
Editing posts costs you small amounts of BTB and spamming threads for bells is lame and I think will get you in trouble so like, don't try and do that. Some easy ways to earn BTB are just to make lots of QUALITY POSTS (long. just talk a lot.) and to welcome people to the forums on the Introduction Board.
Tell ya what Kaai Yuki, add me and open your gates, I'll bring you 500k bells to start you off I got a lot of help on here when I first started and like to help others out