A / B testing is a method to test two similar versions of something and see which version is best. It has been used to test many different things. In the web context, it is often used to test two versions of the same website to see which is best. This is typically done by having a portion of its web traffic go to version A and another part of the traffic going to version B.
This feature is now available in Azure. Through PowerShell, hoping that it will enter the portal in the future, you can describe what percentage of web traffic that will go to the various versions of a website. The different versions of the website is done by using the Deployment Slots feature in Azure Web Sites.
You can put up more than two versions of its website. (This is a small deviation from the description of a “right” A / B test.)