Bells are the currency in the Animal Crossing series. They come in increments of 10, 50, 100, 1,000, and 3,000. They are used to buy objects from stores across the player's Animal Crossing town. They look like Coins, and not actual bells. You can obtain Bells by doing multiple different things, including tree shaking, selling, digging them up, and multiple other ways. Having 99,000 bells automatically fills a bell bag, used to store bells. So far, Bells have appeared in every iteration of the Animal Crossing series including Animal Forest, Animal Crossing, Animal Crossing: Wild World, and Animal Crossing: City Folk.
Ways of getting Bells
Sell your items, almost anything can be sold, whether it be furniture or even shells.
Shake trees; You'll find many 100 bell bags just by shaking them. Watch out, though, sometimes you'll encounter a bee hive.
You'll notice many large boulders around your town. What use are they? Well, every day, one rock will have thousands of bells inside of it. The more you hit the rock within a small time limit - the more bells you'll obtain.