A C program, whatever its size, consists of functions and variables. A function contains statements that specify the computing operations to be done, and variables store values used during the computation. 

C functions are like the subroutines and functions in Fortran or the procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions whatever names you like, but "main'' is special - your program begins executing at the beginning of main. This means that every program must have a main somewhere.
main will usually call other functions to help perform its job, some that you wrote, and others
from libraries that are provided for you.

The first line of the program,
#include <stdio.h> tells the compiler to include information about the standard input/output library; the line
appears at the beginning of many C source files

One method of communicating data between functions is for the calling function to provide a
list of values, called arguments, to the function it calls. The parentheses after the function name
surround the argument list. In this example, main is defined to be a function that expects no
arguments, which is indicated by the empty list ( ).
#include <stdio.h>                                 include information about standard library
main()                                                       define a function called main
{                                                                
                                                                   that received no argument values
                                                                   statements of main are enclosed in braces
          printf("hello, world\n");            main calls library function printf
                                                                   to print this sequence of characters
 }                                                                \n represents the newline character