A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
According to VBA HELP,
The Null value indicates that the Variant contains no valid data. Null is not the same as Empty, which indicates that a variable has not yet been initialized. It is also not the same as a zero-length string (""), which is sometimes referred to as a null string.
Here are some interesting values, which may help your understanding a bit better
range("a1").Clear
?isnull([a1])
False
?[a1]=""
True
?worksheetfunction.CountBlank([a1])
1