A NULL value takes up one byte of storage and indicates that a value is not present as opposed to a space or zero value.
A NULL is not a space or empty string or a zero. But some value undefined telling that value is not present.
From front End we can send the value as NULL by DBNull.Value
And in SQL SERVER it can be searched by following query
Select * from dbo.DimCustomer where Suffix IS NULL
here it will return the rows with Suffix column containing a NULL means Noting.
Reference : Dilip Kumar Jena ( https://sqlexplore.wordpress.com )