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A pointer to a constant is a pointer that points to a memory location containing a constant value, meaning that the value at the memory location cannot be modified through this pointer. However, the pointer itself can be modified to point to a different memory location. Note that this does not mean that the value itself is a constant in memory, it is just constant in the context of this pointer. In C, a pointer to a constant is declared using the `const` keyword before the data type: ```c const int *ptr; ``` Here, `ptr` is a pointer to a constant integer. It can point to any integer, but the value of the integer cannot be modified using this pointer. For example: ```c int num1 = 10; int num2 = 20; const int *ptr = &num1 // OK *ptr = 15; // Error: assignment of read-only location ptr = &num2 // OK ```

A constant pointer is a pointer whose memory address cannot be changed after it has been initialized. This means that the pointer will always point to the same memory location throughout its lifetime. The value at the memory location pointed to by the constant pointer can still be changed, if allowed by the type of the pointed object. In C, a constant pointer is declared using the `const` keyword after the asterisk: ```c int * const ptr = &some_variable; ``` Here, `ptr` is a constant pointer to an integer. It must be initialized when declared, and it cannot be changed afterwards. For example: ```c int num1 = 10; int num2 = 20; int * const ptr = &num1 // OK, initialization required *ptr = 15; // OK, can modify the value of num1 through ptr ptr = &num2 // Error: assignment of read-only variable 'ptr' ```

In summary, a pointer to a constant prevents the modification of the value it points to, while a constant pointer prevents the modification of the pointer itself after initialization. The two concepts can also be combined, resulting in a constant pointer to a constant: ```c const int * const ptr = &some_constant; ``` In this case, the pointer cannot be changed, and the value it points to cannot be modified either.