What is a tubal pregnancy?

I was looking around Answers for stuff on periods :( and kept coming across things called tubal pregnancies. As I understand, it can be a life or death situation. I'm 13, and am wondering what it is. Can anyone tell me? I have no idea what it is, and obviously am not planning on doing anything once I get an answer...????? (I really don't want to sound stupid, but, well...)