It's an european clothing retailer with shops placed all around the world. They sell cheap and nice clothes, hence the popularity among teenage females. It originated in Sweden.
Did you know that H&M is more correctly pronounced: "Hennes och Mauritz"? That's 'cause "och" means "and" in the swedish language. A common mistake is that people say "und" instead, which isn't right as it's a german word, not swedish.
Many non-european people don't have a clue how to pronounce it that way, so just feel free to say "Hennes AND Mauritz", or just "H&M" so you don't end up making yourself look stupid.