Mockito is a mocking framework. It is a Java-based library used to create simple and basic test APIs for performing unit testing of Java applications. It internally uses the Java Reflection API to generate mock objects for a specific interface. Mockito is in top 10 Java library across all libraries, not only the testing tools.
JUnit is a unit testing framework for the Java programming language. JUnit is used primarily to test each and every unit or component of your application, like classes and methods. It helps to write and run repeatable automated tests to ensure your code runs as intended.