Not Shutterstock, but I'm an iStockphoto contributor. If you really want to get into stock photography you might consider them since they're owned by Getty and it's a way to get in the door there.
As far as how it works... at iStockphoto you have to submit images and get approved as a contributor. Then you post images, which each have to go through their approval process, and then wait for people to buy them. It's difficult to get common topics through the approval process because they already have so many photos of a kid eating an ice cream cone, dogs playing frisbee, lightbulbs, etc... that they only accept really clean, well executed work. It's much easier to get more conceptual images approved. There are a lot of guidelines in regards to model releases, brand names in photos, and things of that nature. They also take editorial images, which are a little easier to get approval for.
As far as money made, the more you submit the more you make. If your work is outstanding you can make a living. I know a couple of people who do so it is possible. If your work is just average you might make enough over a few years to cover the time you spent shooting and editing the image. If your work is below average you won't make it past the screening process.