Question:

What is the difference from NULL and "0"?

Example:

return NULL;
return 0;

Answer:

Conceptually, zero (0) is a number, and NULL is a value that represents "no value". As such, 0 can be added, subtracted, etc., but NULL cannot. The NULL value for a variable can indicate, for example, that a variable has not yet been assigned a value.
The semantics of NULL may vary by language or application.