Empty string vs NULL

I've a table where some rows have some blank cells. I tried to select such rows using IS NULL function. But the query select 0 rows.

select * from zzz_fkp_registration_female where fname is null;
(0 row(s) affected)

Now I changed my query to:

select * from zzz_fkp_registration_female where fname is null or fname ='';

I got the desired result.

Why is IS NULL not giving the result? What is the difference between IS NULL and '' since the cells are empty. Please explain.

Some differences between them:

• NULL can be assigned to any type, as opposed to empty string which won't be compatible with date/numerical fields.
• NULL is an UNKNOWN value, it doesn't have a value as opposed to an empty string, which is a value, but empty one.
• As far as I'm aware of, NULL shouldn't capture memory as opposed to an empty string which does.
• null = null will result in null as opposed to ''='' which will result in TRUE.