It depends on how the cell or field is formatted. In theory Null means the cell is empty - ie. does not contain anything.
In practice, if the cell is formatted as Text an empty cell contains an empty string which can be referred to as "".
If formatted as a number, the empty cell can contain either nothing - ie Null, or number zero. This is the one that usually gives a problems - especially when using MS Access - because the system does not count a Null cell as zero, so calculations go wrong.
In Excel this is not a big problem unless we are writing macros that search for zero. A way of getting round this is to actually put zeros into empty cells.
MS Access has a nz() function for use where this might be a problem.
