Second Life is a bit like a "real life simulator". People control a so-called avatar, a virtual human that they make to look like themselves or their ideal self perhaps.
What people do in there mostly is socialize, just chat with eachother basically. But as people who socialize in the real world, people in Second Life also care about how they look, how others look, where they go, and so on. So people get smart clothing for their avatar, pretty houses in interesting neighborhoods, they go to clubs, restaurants, or whatever they fancy.
Second Life has a currency, Linden Dollars, that can be exchanged with real world currency. You want a rich avatar, you buy a lot of Linden Dollars with your real life money. If you through trading or providing services make a lot of Linden Dollars, you change them back to real world currency.
For example, an area with many interesting restaurants, a pretty park, a high traffic of interesting people, living near that is attractive, so land here is expensive.
Or running a business there is attractive. Someone might buy a piece of land there, hire an architect to design a pretty building and an interior decorator and so on. He'll charge money for entering the club. He'll hire good DJs so they play the coolest, hippest, newest music when people come there.
It's a real world social life simulator, with all the monetary implications of that.
Companies establish themselves in there too, and provide services or just get marketing. For example Toyota sells very cheap cars in there, not only do they get some advertising effect from it, they also allow the buyers to change their cars, and the changes people choose, the real world Toyota car designers use as input on what people really want their cars to look like.
There are lots of job opportunities in Second Life that will make you real world money if you're good at it, real estate investor, clothes designer, architect, DJ, events organizer, club owner, etc.
Pretty strange, but when you think about it, why shouldn't people who spend a lot time chatting and making friends online want to do and act exactly the same way as when they socialize in the real world?