A function is a self contained block of statements that have predefined meanings.you can  divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is so each function performs a specific task.

A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

Defining a Function:

return_type function_name( parameter list )

{ body of the function }

• Return Type: A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Function Name: This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Function Body:The function body contains a collection of statements that define what the function does.
• example of function

• #include <stdio.h>
  
  int mult ( int x, int y );
  
  int main()
  {
    int x;
    int y;
  
    printf( "Please input two numbers to be multiplied: " );
    scanf( "%d", &x );
    scanf( "%d", &y );
    printf( "The product of your two numbers is %d\n", mult( x, y ) );
    getchar(); 
  }
  
  int mult (int x, int y)
  {
    return x * y;