A JUnit fixture is a set of objects that are used as a baseline for running tests. A fixture can be anything from a single object to a complete system. The purpose of a fixture is to ensure that tests are repeatable and that test results are consistent. For example, consider a test that checks the functionality of a login system. The test might need to create a user, log the user in, and then check that the login was successful. In order to create a repeatable test, we would want to use the same user each time. This would be our fixture. Using a fixture has several benefits. First, it makes tests more reliable because we know that the baseline objects are always the same. Second, it can make tests run faster because we don't need to set up the fixture each time we run the test. Finally, fixtures can make tests easier to read and understand because they provide context for the test steps. JUnit provides two mechanisms for creating fixtures: @Before and @After methods, and static methods annotated with @Rule. @Before and @After methods are instance methods that are called before and after each test method respectively. This gives us granular control over how the fixture is set up and torn down for each test method. Static @Rule methods, on the other hand, are executed once per test class. This can be useful when we want to share data between test methods or if we want to perform setup or cleanup tasks that are not specific to any one test method. No matter which mechanism we choose, fixtures are an important part of writing repeatable and reliable JUnit tests.