A/B testing is the act of simultaneously testing specific elements of your marketing to determine which performs or converts best. The purpose of A/B testing is to continuously improve the performance of your marketing in order to increase your success in converting visitors and leads into customers.
Once you decide what to test on your landing page, email, blog or web page (e.g. headline, Call-to-Action, header image, etc.), you can create two or more versions of each. To accurately assess performance, you will want to equally split traffic (for a landing page or webpage) or recipients (for email) between each version.
Change only ONE element at a time, whether that be your headline, body copy, layout, images or Call-to-Action. If you test multiple elements at the same time, you will complicate your ability to determine which element triggered a change in response.