YoVille is a virtual world for teens and adults, and it has 5.5 million Facebook users per month. You build yourself an avatar, usually one that somewhat resembles you (except that it has a giant head and a tiny body). At first, your character has a limited amount of clothes and accessories it can wear. That'll change as you accrue virtual money.
Your YoVille character starts in its own apartment, and with your mouse you can move it around, by clicking on doors. As the name suggests, your character lives in the town of YoVille, which has a bank, gym, casino, diner, nightclub, shops and even an animal shelter, if you'd like your avatar to get a pet. A factory on the outskirts of town provides your character with employment -- the more money you earn, the more stuff you can buy.
If your friends have the application installed, you can socialize with them in YoVille. Or you can make new friends in YoVille by walking up to someone else's character and initiating a chat. If all goes well, you can add that person as a buddy. Your friends can visit you in your apartment, or you can meet up at the diner or nightclub. Your game of YoVille can continue indefinitely. It's constantly adding new features and locales to the game.
Your YoVille character can live a solitary life. But it's better if you start or join a "crew." The bigger your crew, the more money you earn at work, which means you get to buy more all-important stuff.