Most zines revolve around a music scene of some sort, but others are dedicated to artwork, poetry, cartoons, editorials and short stories. Because zines do not have any sort of corporate backing, they are very rugged, individualized, and much more charismatic than larger, more popular magazines whose content is often dictated by their advertisers.
So it's basically a crudely made magazine.
Mostly punk/poetic/indie/misunderstood/lost/artsy/goth/loner/whatever teenagers do it now.
They write/draw whatever they want on pieces of paper, photocopy, staple, and sell them for anywhere from a few dimes to a few bucks. Many are also free.
If you buy a zine for over $2, you got ripped.