Codeception (home) is a modern, powerful PHP testing framework written in PHP.
It comes with a number of modules and extensions that are comparable to WordPress plugins and themes.
Modules and extensions are combined in suites to be able to run a specific type of test. Each suite will handle a specific type of test for a specific set of code.
wp-browser is none other than a collection of modules and extensions for Codeception made specifically to test WordPress applications.
PHPUnit is the most widely known PHP testing framework. As the name implies it was born to make unit testing of PHP code easier but its scope and power has grown well below that.
Codeception is based, and uses, PhpUnit to wrap some of its functionalities into an easy-to-use API.
The two are so compatible one with the other that Codeception can run PHPUnit tests with little to no changes.
This documentation will not cover this subject and will only deal with Codeception-native test methods but you can find more information here.