This document consists of three sections:

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 a JUnit 4 based Runner for running any TestEngine on the platform in a JUnit 4 based environment.
JUnit Jupiter is the combination of the new programming model and extension model for writing tests and extensions in JUnit 5. The Jupiter sub-project provides a TestEngine for running Jupiter based tests on the platform.
JUnit Vintage provides a TestEngine for running JUnit 3 and JUnit 4 based tests on the platform.

Already consulted the JUnit 5 User Guide?

Aggregates all JUnit Jupiter modules.
Defines JUnit Jupiter API for writing tests.
Provides the JUnit Jupiter TestEngine implementation.
Support for migrating from JUnit 4 to JUnit Jupiter.
JUnit Jupiter extension for parameterized tests.
Common APIs and support utilities for the JUnit Platform.
Support for launching the JUnit Platform from the console.
Public API for test engines.
Provides Java Flight Recorder events for the JUnit Platform.
Public API for configuring and launching test plans.
Defines the JUnit Platform Reporting API.
Runner and annotations for configuring and executing tests on the JUnit Platform in a JUnit 4 environment.
Aggregates all JUnit Platform Suite modules.
Annotations for configuring a test suite on the JUnit Platform.
Common support utilities for declarative test suite executors.
Provides a TestEngine for running declarative test suites.
Defines the Test Kit API for the JUnit Platform.
Provides a TestEngine for running JUnit 3 and 4 based tests on the platform.