This are 2 data type qualifier which determines the way the data can be accessed.

Constants

When declaring variable as a constant the keyword const is placed before the data type declaration. Variables qualified this way cnnot be modified while the program is running.

1: const int ciVar = 123;
2: ciVar = 123; //the variable cannot be modified error


Volatiles

When declaring variable as a volatile the keyword volatile is placed before the data type declaration. This tells compiler that variable can be modified any other way not explicitly specified by this program.

1: volatile int viVar; //declaring volatile variable