A JUnit developer creates and executes unit tests using the JUnit framework, a crucial tool in Java for test-driven development. They write scripts to test individual units of source code, functionalities, or procedures, ensuring they are fit for purpose. They are responsible for identifying and fixing bugs, maintaining code quality, and improving the reliability of software applications. They also document testing procedures, analyze test results, and provide feedback to the development team. The goal is to ensure that all tested software meets specified requirements before it's deployed, reducing errors and improving overall software quality.