A/B Testing is a testing method where two or more users each use a different version of the same website to perform the same task. It is a good way to get quantitative data, as analytical data can be taken from each user, such as how long it took to perform a task. Decisions about which version works best can be based on this data.
Advantages of this method include being able to learn how certain website elements are impacting user behaviour. It also enables assumptions to be tested. Companies opinions about what elements make the best experience can be proven/disproven based off this test.
Disadvantages of this method are that feedback is only given based on the available versions. Prior testing must be done to determine what makes a good website, and the test versions must be based off those results.