Animal Crossing is kinda like an offline online game.
Basically no two towns are the same and certain things (like fruit) are unique to different towns. You can mail items to other towns by sending them the the Name and Town name of the person you want to send it to. The game then spits out a really long and complicated password which you can give to the person you intend on sending the item to. All they need to do is type in the code and they'll receive the item.
Obviously you can trade anything from fish, insects, furniture to rare items like NES games and fossils...
It's really an amazing game and I highly recommend it to anyone bored of more conventional genres. I've been playing a lot more of it than Metroid Prime (I got both for Christmas), by a ratio of about 10-1.