A null mutation is a change in genetic sequence that can cause the complete loss of the protein encoded by the gene either by failing to transcribe it to RNA, or by an inability to translate the RNA sequence to protein. In this case no protein is actually synthesized by the organism. This is not the only possible result however: The other potential is the production of a non-functional protein. In this case, the mutation has changed the protein in such a way that it has no activity (if it is an enzyme) or exhibits none of the functions or capabilities characteristic of the native form.
A classic example is in blood types: type O blood is a null mutant arising from a mutation in the allele for type A, resulting in a non-functional protein.
See also: Wikipedia: Null allele