User Defined Functions (UDFs) are custom functions created by users to perform specific tasks in programming. They allow programmers to encapsulate code into reusable blocks, making it easier to manage and maintain. UDFs can take inputs, known as parameters, and return outputs, enabling more organized and efficient code. In many programming languages, such as Python, Java, and JavaScript, UDFs enhance code readability and reduce redundancy. By defining a function once, it can be called multiple times throughout the program, streamlining the coding process and improving overall productivity.