JUnit is an open source unit testing framework for Java and one of the most popular tools used by Java developers. It provides a simple, easy-to-use interface for writing tests and helps developers quickly and efficiently test the code they write. JUnit is widely used for unit testing of applications in the Java environment, and its popularity is only increasing. With its concise syntax, it's easy to learn and use, and its powerful features make it an invaluable tool for developers. JUnit also integrates seamlessly with different Java IDEs, allowing developers to quickly and easily run tests and see the results. For developers who want to ensure their code is working as expected, JUnit is the go-to tool.