Steven, You claim you need a definition of a "good" photograph, and Jeremy's defined some of the mechanical requirements for you, but I'd suggest you go to your local library and check out as many books as you can by Eugene Atget, Paul Strand, Edward Weston, Andre Kertesz, Chim (David Seymour), Henri Cartier-Bresson, Robert Doisneau, Willy Ronis, Brassai, Walker Evans, Elliott Erwitt, Dorothea Lange, W. Eugene Smith, Marc Riboud, Robert Frank, Garry Winogrand, Lee Friedlander, and Steve McCurry. Study the books. It's looking at the work of the masters that tells you what a good photograph is.