What is the difference between adult and teenage books?

What do publishers/editors look for when deciding to label a novel young adult, teenage, adult, ect.?What is the difference between adult and teenage books?
It's mostly about content. If it has a lot of cursing and drugs, then it most likely wouldn't be found in the ';kids'; section or YA. Adult and teenage books tend to sometimes blur the line. And actually, many books can be marketed in many genres. Take His Dark Materials. At BN, you can find the series in Sci/Fi as well as Children's. In that case, it's a matter of different marketing of the covers. As far as sexua content goes, if it is very extravagant, they usually will market it in the adult section. But, with the resergence of Gossip Girl, it is getting a bit more in the teen section as well.What is the difference between adult and teenage books?
Well, I've found that teenage books are enjoyable and easy to read but I like adult books more because the writing is often more mature and there are lines that really just make you question things more instead of just tellnig a story. But that is just my opinion.