A goth is just a person, usually one who appreciates both the "positive" and "negative", the "yin" and "yang", "good" and "evil" aspects of life. Usually goths believe that there's nothing in this world that can be completely spelled out in black and white. Usually goths tend to wear black clothing, ornate with metal, but also often enjoy interspersing it with bright colours. A lot of goths dye their hair or have piercings, or wear lots of makeup... which is more an artistic outlet than anything. I guess goths just focus on the dark side of things, because to do so is to be on sort of the "other side" of most of society, which they do enjoy, regardless of what they say. Often goths shop at the hot topic, which doesn't automatically define one as a poseur. Nor does it make someone gothic to shop there. Gothic music doesn't make much of a presence in society these days, though darkwave/industrial/synthpop/black,death metal are often mistaken for "gothic" music. Music that is actually gothic? Joy Division, Siouxsie and the Banshees, Bauhaus, The Cure, etc. These bands are listened to by many people who are not gothic, and no one at the time really labelled it as such. Gothics are just members of an almost completely undefined subculture. Gothics are generally good, benevolent, empathetic, artistic, intelligent, well-read people.
There is the odd asshole, but the same can be said for every group. Gothics almost always can appreciate all kinds of music. Many gothics enjoy listening to Enya. Many gothics enjoy listening to Motorhead. Many gothics enjoy listening to New Order. See the point? There is no real definition, but Gothics is Gothics. We just are what we are, and we only ask for a bit of kindness and understanding, even though we are just a bit strange. =)
Examples of gothics:
Pure goth: Usually found dressing in sort of old-school european style clothing, with lots of black. Often found listening to Sisters of Mercy, Bauhaus, etc. Often is a strong supporter of animal rights, or a vegan. Usually subscribes to some form of paganism.
Usually wears clothing that depicts a post-apocalyptic Mad Max style motif, often seen with a trenchcoat. Most often have a "harder" personality, but unlike pure "Rivetheads" the classic industrial club-goer, they tend to be a bit less immature and full of rage. Often found listening to music like Nine Inch Nails, Switchblade Symphony, Skinny Puppy.
Perkigoff^_^: This is the kind of gothic who often wears a lot of clothing that is considered gothic, but conveys a lot less serious and sombre attitude. Can be found listening to just about anything, and often this person is more of a "weekend" goth... i.e. they work normal jobs and generally have a lot of friends that are not goths, as well as friends who are. This is I have to say the most common type of goth.
Common Goth: Sort of a cross between all of the above. Most goths dress gothically on a very regular basis, unless at work. Most goths are generally very happy, friendly people. Most goths are also very somber at times.
In other words, that was useless babble! But hopefully it's helpful to you.