JUnit is a unit testing framework for Java that allows developers to write and run repeatable automated tests of Java code. Key aspects of JUnit include: 1. Tests are written by creating test classes that extend JUnit's TestCase class and contain test methods with names beginning with "test". 2. Tests use JUnit's assert methods like assertEquals to make assertions and determine whether the test passed or failed. 3. Annotations like @Before and @After allow setting up fixtures that run before and after each test. 4. The TestRunner class collects and runs all test cases and reports results. 5. Tests can be organized into test suites to group related tests and control execution