LinkedIn is basically a way to network with others. You might have a list of people you know, but LinkedIn allows you to keep those contacts in an easy to use yet formal way. On top of that, you can meet others through the people you know.
LinkedIn uses the concept of connections. If I know you, we can make a formal connection between each other on the site. So let’s say that I make a connection with the CEO of a game company, and you are interested in meeting that CEO for a business proposition. If you know me, you can send a message to the CEO through me, and I will forward the message on for you. Naturally, if I don’t know you or don’t think it would be good to pass the info on, I won’t. But connections are made quickly, and you can grow your personal network to discuss business opportunities, gain expertise, or just keep contact with old coworkers.
I’ve long known that I needed to discuss game development with a mentor or two. I’ve just signed up, but I’ve already made contact with a few Chicago-area game developers that I didn’t know before. It’s still early to say, but LinkedIn might be one of those tools that you can’t live without. The Beta Subscription is currently free, and I think the people behind LinkedIn are trying to figure out how much to charge in the future or if they should use advertising. Part of subscribing for free is to fill out a survey to help them decide.