#include<>, include the predefined header files, but
#include"...." include the userdefined header files in c
program.  in this within "...." we mention the path where
the users header file is stored.

Volatile variable will be used when we are dealing with 
external memory, external device and interrupt service 
routine. In these situation if some variable needs update 
or gets update without user knowledge in that case variable 
should be declared as VOLATILE. otherwise we may loose 
these update.