This is a repost of an article I wrote back in 1999, but I thought it might be interesting to recycle it, since it still seems relevant. I did edit it a little – I wrote it after the birth of my second child, who is now 11, so it did not make sense to refer to him as a baby ;-). Here it is:
A null value is a missing value. Yet a value that is “not there” may provide more information than one might think, because there may be different reasons that the value might be missing. A null value might actually represent an unavailable value, that the attribute is not applicable for this entity, that there is no value in the attribute’s domain that correctly classifies this entity, etc. Or the value may actually be missing!