What is Human Growth Hormone?

EelBait asks: "I get a lot of spam selling Human Growth Hormone (HGH). Supposedly this stuff builds muscles, make one lose weight, enlarges various external organs and glands, etc., etc. Does anyone really know any good articles about this? Is it dangerous? The fact that it's being sold through spam is enough to give me pause, but I want to know if there are any legitimate uses for this stuff."