An A/B test, also known as a split test or experiment, is used to determine which version of a user-visible change is more effective. It works by showing half of users a new behavior and half an old behavior, and then comparing the results. The test can be extremely easy to use, and it's recommended to be used on every user-visible change. The framework uses a simple Python code snippet that includes a call to `h.branch`, which determines whether to show the default or new version of a page. The test continues until a clear winner is determined, at which point the winning behavior is implemented for all users. The test can also be paused and resumed by an admin control, allowing for flexibility in how it's run.