I only recently understood A/B Testing when my boss asked me to work on email marketing campaign. We signed up for mailchimp (free account, those guys are awesome), and after a reaching some milestone on using the application. I unlocked the A/B split testing tool on the application.
Basically mailchimp A/B testing for email newsletter helps you test which subject lines work best.
A/B testing is testing an original idea vs. a new version of that idea to know which works best (sells more, as the case may be).
But if you still dont understand all my rambling then you want to watch this brilliant cartoon about A/B testing from the folks at VisualWebsiteOptimizer.