Please refer C – Constants topic in this tutorial for more details on const keyword.
2. volatile keyword:
When a variable is defined as volatile, the program may not change the value of the variable explicitly.
But, these variable values might keep on changing without any explicit assignment by the program. These types of qualifiers are called volatile.
For example, if global variable’s address is passed to clock routine of the operating system to store the system time, the value in this address keep on changing without any assignment by the program. These variables are named as volatile variable.