A/B testing is an easy way of comparing two versions of a page to know which converts better. Multivariate testing on the other hand compares multiple changes of possible web design and can be expensive and complex.
How does do the two compare
A/B requires little development and can be implemented by SMB’s with limited budget and technical knowhow. You can get a crowd source web page design at very low cost and use Google analytics to setup the tracking parameters.
A/B compared to multivariate testing – http://www.smashingmagazine.com/2010/11/24/multivariate-testing-in-action-five-simple-steps-to-increase-conversion-rates/