You need something that runs your tests. JUnit is a framework that lets you define unit tests in your Java code and run them.
JUnit belongs to a family of testing frameworks collectively called xUnit. The SUnit is the grandfather of this family and was designed by Kent Beck in 1998 for the Smalltalk language.
While JUnit is specific to Java, the ideas are sufficiently generic for ports to have been made in, for instance, C#. The corresponding test framework for C# is called, somewhat unimaginatively, NUnit. The N is derived from .NET, the name of the Microsoft software platform.
We need some of the following nomenclature before carrying on. The nomenclature is not specific to JUnit, but we will use JUnit as an example to make it easier to relate to the definitions.