In the series of "Basic Concept And Terms" (yup, I know, John is well known for misspelling people's last names, including mine), several people have already chimed in with their own definitions of the "gene", demonstrating how unclear this concept is and how much disagreement there is among the practitioners depending on the type of research they are doing (e.g, molecular biology, developmental biology, population genetics, evolution, etc.).
See how the term was defined and explained by PZ, Sandra and Greg so far and you'll see those differences in emphasis.
Now Larry Moran joins the fray with one post on what a gene is not (though many erroneously cling to this definition) and one post on what a gene is, at least from Larry's perspective. Good reading altogether.