What is the difference between a gene and an allele?

A gene is a section of DNA or RNA that codes for a protein. Alleles are different versions of the gene.  For example, if a gene coding for a particular enzyme mutated, then there would be two alleles of the gene: the 'normal' form and the mutated form. The proteins encoded would probably also be slightly different, for instance the mutated allele might code for the same enzyme but in a slightly different shape.