A follower/fan of music that was different/older/barbaric compared to others. There is really no specific type, as pop can also be included. There is also not necessarily much else in common, as with any other group. A "goth" can be anyone, assuming they enjoy the music described.
Misuse has also applied the term to metalheads, industrial fans, and the "suicidal" yuppie children. The bands currently labeled incorrectly as gothic include Marilyn Manson, Slipknot, and most nu-metal, or industrial-metal hybrids.
Joy Division were gothic compared to the rest of the music scene.