A Primer on A/B Testing
A great introduction to A/B testing from the folks at A List Apart.
A/B testing is used in many instances, from development to advertising and is essentially developing two different web pages and seeing which users respond to best. From that you can then roll out the most sucessful version. A/B testing helps to minimize the impacts of testing different designs, content, etc on users without directing all users there and you seeing a direct impact on your bottom line.
In the end the data gathered from A/B testing is going to help you make informed decisions about your product / site to make it better.