Multivariate Analysis is used to test multiple options at once. These multiple things could be 5 different images on a webpage, 3 different background colors of the homepage of your site or 3 different body copies for a particular offer.
Offermatica is a vendor which was acquired by Omniture which is known to help most of the clients and agencies to do multivariate analysis. One of the examples of their website does a very good job of explaining how and why this kind of Analysis is very crucial in taking the correct decision while making any changes on a website.
Offermatica also has a demo which explains the A/B testing for various elements on the webpage