Explain pointer to the constant and constant pointer ? What is the difference between them ?

Pointer to constant does not allow to change the value at the address pointed by pointer ( *ptr).

1. const int *ptr = 3;
2. int const *ptr; // 1 and 2 both are having same meaning.

*ptr = 5; // not allowed

Constant pointer does not allow to change the address to which pointer is pointing.
int i = 10;
int * const ptr = &i;
int j =80;

ptr = &j; // not allowed.

constant pointer to constant data
int i = 9;
const int * const ptr = &i;
int j =90;

*ptr =10; // not allowed
ptr = &j; // not allowed.