First, I know Swagbucks seems like a total scam. I resisted joining the site for months for that exact reason. Then I gave it a chance and I have earned more than $200 worth of gift cards in the past few months.
This is how it works. You register at Swagbucks.com and the the site installs a small toolbar at the top of your Internet browser (such as Internet Explorer or FireFox). When you search using the Swagbucks toolbar, you can win swagbucks, which you can redeem for prizes and gift certificates.
Some searches will earn you one, two or five swagbucks. For others, you'll get nada. Today is mega swag bucks day, when the site awards 10, 20, 50 and 100 swagbucks for searches. If you're going to try Swagbucks out, today is a good day to join.
Once you accumulate some swagbucks, you can turn them in for prizes. I always go for the gift cards. For example, 45 swagbucks will get you a $5 Amazon gift card. 130 swagbucks will get you a $10 Starbucks card, and so on. You can trade in your swagbucks for clothes, MP3s, movies, and more.