JUnit is used for unit testing. And it is used for other types of testing as well.
I don’t think of JUnit as a Unit Testing Tool. Instead, I view it as a way of running code marked as ‘runnable by JUnit’. That code is marked by @Test annotations and is often called a Test, and those ‘Tests’ are often used for Unit testing, but they don’t have to be.
JUnit can be used as a test runner for any kind of test: e.g. system and integration tests; tests which are interacting with a deployed application.