Goths are undefinable as a group. Though dark clothes and somber attitudes once defined them... now they've broken through that stereotype, as well. However, I found the following definition at Urban Dictionary:

Someone who likes the darker side of things. They usually listen to death metal and goth music, such as Dismember and Bauhaus. Marilyn Manson is NOT goth. He's just...weird. Real goths are not depressing and suicidal like the posers you see at Hot Topic. Goths are fun to be around and aren't afraid to laugh at themselves every so often. They don't worship Satan and aren't evil despite what some ignorant people might say. Goths don't all dress alike either. They like to create their own unique style. Goths are also very intelligent and creative. A lot of them are writers and artists. Goths DON'T envy the popular people. They are what they are because it makes them happy. Goths would rather stay who they are and be among the so called "freaks" than be like everyone else and be popular. They understand that there are more important things in life than popularity, and usually succeed in life while the popular ones don't become much of anything.

If someone becomes "goth" because they couldn't get any friends or are depressed and angry all the time, they are most likely a poser.