What is an A/B test?

A/B test is a type of broadcast in which a list is split into 2 parts. The first part defined as testing group will be further divided into equal segments each  receiving different creative content. The other list – the remaining part of addressees receive the best-performing version of the test. This technique enables to measure the performance of every message element in terms of conversions and determines which is more effective.

The manual on creating an A/B test can be found here.