Now, before I begin, I grant that I tend to fall in line somewhere more on the goth side of things, which is a different subculture altogether from punk, but I believe I can still answer your questions.
Why do punks like skulls? Well, punk started as a rebellious movement, and it was meant to draw attention, be loud, be against the grain, and be against the conservative political era in which it was born. Skulls represent death, and they wanted to break away from their environment, to put it to a certain kind of "death" in order for the phoenix of society to be reborn from the ashes of the violent conservative government and into something more accepting.
Why is spiked hair a common style? Because it draws attention to itself, especially when brightly colored. Originally, punks wore more dirty clothes, purposely torn, sometimes would get into fights just to have some real blood on that. I suppose the spiked hair was meant to look roughed up, deranged even.
Why with the black eyeliner? This isn't so much of a punk thing, but as for goth fashion, it draws attention to the eyes. Goths tend to be incredibly vain, and their elaborate make up is meant to be an expression of self. I find that most men prefer eyeliner only on the under eyelid, which usually gives a tired or deathly look, which is something a lot of goths go for.
What concept are they trying to present? For punks, it's all about rebellion, being broke, surviving through violence, and spitting in the face of the world that puts them through what they've gone through. They're innately fighters, of a sort. That's what they seem to be presenting.
What kind of music do they like? Punk, obviously. Detroit Metal City isn't something I'd classify as punk culture, but rather, metal culture, and it was satirized for comedic effect. Now, as for what constitutes punk music exactly, you might wanna talk to a guy around here called Paipis. I will say that typically the lyrics are politically charged and/or rude.
Since I don't know what you mean by weird cars, I'll continue to the next question.
What kind of hobbies do they have? That's a tough one. I've found some crossover between punk and skateboarder culture, although that likely came about later. I've also heard that some punks even started some graffiti movements, which doesn't surprise me really. I mean, this is a pretty hard question to answer, since under the subculture, they're all still people with their own interests.
Anyway, I answered the questions as best as I could from a "punk" perspective, although I could easily comment again for goth culture, and even for some of its subcultures, since I am vastly more familiar with goth and its family tree.