When we talk about conversion rate optimisation, we are usually referring to running split tests. A split test is one where we show different versions of a web page to groups of users and determine which one has performed better. We can run A/B tests. Here each version being tested is different from all the others. A/B tests often involve just two versions being tested: the original and an alternative. There can also be many alternatives being tested.
A/B tests are ideal for an initial foray into conversion optimisation, as they can be easy to set up. If you are running just one alternate and the original, it can also mean that you get a quicker result.