They can be used to buy animal crossing items easier, without dropping bells on the ground for hours. They can also be used to buy stuff for your profile on this website.
TBT (Bell Tree Bells) are a forum of currency used on the forums to buy/sell art, in-game items, collectibles (they are the small icons under avatars), etc. Unfortunately, since I'm on an iPhone, I can't give you direct links to some guides. However, you can go over to the Bulletin Board and in the sticked threads (they will appear as yellow) you can find "The Bell System: Beginner's Guide" thread.