In C programming, a function is a set of statements that performs a specific task and generates an output, structured as 'return_type function_name (list of args) { body of the function }'. Function definitions and declarations are vital, with examples given for both, showcasing how to declare and define a function such as 'int product(int a, int b)'. Functions can be categorized based on their arguments and return values, including library functions and user-defined functions.
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