Difference between null and recessive allele?
To add to Remi b's answer - this question is confusing because 'null' and 'recessive' are terms emerging from two very different levels of analysis. The concept of 'recessive' existed before we knew what genes were, or how they worked. It just describes the patterns of inheritance you see in a gene's effects.

The concept of a 'null' allele however came later, and explicitly describes how an allele works - by destroying gene activity. So 'null' is molecular concept, 'recessive' is an abstract genetic concept. It so happens that null alleles are very often recessive, because usually one working copy of a gene is fine. But the two terms are describing genetics at very different levels.