Is a NULL value same as zero or a blank space? If not then what is the difference?

Null is an absence of a value. An empty string is a value, but is just empty. Null is special to a database.
Null has no bounds, it can be used for string, integer, date, etc. fields in a database.
NULL isn’t allocated any memory, the string with NUll value is just a pointer which is pointing to nowhere in memory. however, Empty IS allocated to a memory location, although the value stored in the memory is "".