What's the difference between a null pointer and a void pointer?

Null pointer is a special reserved value of a pointer. A pointer of any type has such a reserved value. Formally, each specific pointer type (int *, char * etc.) has its own dedicated null-pointer value. Conceptually, when a pointer has that null value it is not pointing anywhere.

Void pointer is a specific pointer type - void * - a pointer that points to some data location in storage, which doesn't have any specific type.

So, once again, null pointer is a value, while void pointer is a type. These concepts are totally different and non-comparable. That essentially means that your question, as stated, is not exactly valid. It is like asking, for example, "What is the difference between a triangle and a car?".

They are two different concepts: "void pointer" is a type (void *). "null pointer" is a pointer that has a value of zero (NULL). Example:

void *pointer = NULL;

That's a NULL void pointer.

Void refers to the type. Basically the type of data that it points to is unknown.

Null refers to the value. It's essentially a pointer to nothing, and is invalid to use.