Functions are fundamental building blocks in programming that allow you to encapsulate a set of instructions into a reusable block of code. They can perform specific tasks and return values when called. Functions are defined using the def keyword in many programming languages, followed by the function name, parentheses for parameters, and a block of code that constitutes the function body ¹².

Functions can take parameters, which are variables passed into the function to customize its behavior. If no parameters are provided, default values can be used. When a function is called, the code within it executes, and it can return a value using the return keyword. If no return statement is specified, the last evaluated expression will be returned implicitly ¹²⁵.

In summary, functions enhance code reusability and organization, making it easier to manage complex programs by breaking them down into smaller, manageable pieces.