This document consists of four sections:

Platform
  The JUnit Platform serves as a foundation for launching testing frameworks on the JVM. It also defines the TestEngine API for developing a testing framework that runs on the platform. Furthermore, the platform provides a Console Launcher to launch the platform from the command line and the Suite Engine for running a custom test suite using one or more test engines on the platform
Jupiter
  JUnit Jupiter is the combination of the programming model and extension model for writing JUnit tests and extensions. The Jupiter subproject provides a TestEngine for running Jupiter based tests on the platform.
Vintage
  JUnit Vintage provides a TestEngine for running JUnit 3 and JUnit 4 based tests on the platform.
Other Modules
  This section lists all modules that are not part of a dedicated section.

Already consulted the JUnit User Guide?