Constant pointer:If a pointer is pointing to a constant whose value(constant)can't be changed.

Eg:Consider the declaration int a[10];

here a is a pointer to the base address of the array say 1000.When we try to modify  a value say a++ or a-- it will give error because a is a constant pointer to the array.

pointer constant:If a pointer doesn't to points to any variable,array,structure,object,structure,etc...such pointer is known as pointer constant.

Eg:The well known example for pointer constant is the predefine Macro NULL.

NULL is a pointer constant which doesn't to points to any thing.