What does it do?

PHPUnit is a TYPO3 extension which provides:

1. the PHPUnit unit-testing framework by Sebastian Bergmann
2. a framework for creating database records, faking a front end, creating dummy files and many more useful things
3. vfsStream (a virtual file system)
4. a back-end module to run unit tests for extensions of a TYPO3 installation
5. a CLI test runner which can be used to run the tests from the command line, in a continuous-integration environment, or from within an IDE like PhpStorm