What does a 12" mix of a song mean and how is it different then the original verison?

What does a 12" mix of a song mean and how is it different then the original verison?|||the 12' mix is usually a non radio, longer version that is played in nightclubs or in foreign regions. also might be the version you hear of the song 20 years from now on the radio instead of the standard 'radio edit'. its a DJ thing.