there's really nothing to it....
Basically you live in a town full of animals. There are about 100 different animals that randomly get anywhere from 5-15 of living in your town at one time...each with different personalities and catchphrases. You've got a house that constantly needs upgrading and paying off, and in order to do this you must go around running errands, catching fish, catching bugs, and finding other ways of making money.
The coolest thing is, everything is done in real time. The fish and bugs you can catch are different in winter at midnight than those during summer in the day. There are tons of things to collect and play with, and tons of things to do. It's really quite charming and once you get hooked on it its easy to waste gobs of time on.
There's really no end to the game...it's just a way to pass the time because there's always something new to be done....catching a rare fish, planting trees, growing hybrid flowers, getting the animals to say "d-bag" to everyone...it's neat.