The document discusses functions in C programming. It defines a function as a block of code that performs a specific task. There are two main types of functions: predefined and user-defined. User-defined functions are created by the programmer to perform custom tasks, while predefined functions are part of the standard library. Functions can be classified based on whether they accept parameters, return a value, or both. Well-structured programs use modular design with separate functions to decompose complex problems into smaller pieces.