A/B Testing, also known as split or bucket testing, is the process of comparing two versions of a webpage, app, mail,  ad to see which one is performing better, etc.  It tests a control (A) version against a variant (B) to measure which one best resonates with your target audience.

You can test website/app, layouts, email subject lines, designs, ads, CTA button, text, etc. Users are randomly shown either of the two versions, and then important metrics such as click-through rates, conversion rates, time spent, etc. are tracked to analyze the performance.  It plays a very important role in campaign management and helps marketers to make data-driven decisions.