Science Question...and you thought you were done with those

So I met the matriarch of Warren's family, Grandma Corey, and the subject of the Northern Lights came up at dinner. Here is the question:
What causes the Northern Lights and why can they only be seen in certain parts of the world? What are those parts of the world?
Okay and another Science question. Are we supposed to keep posting answers? I haven't looked at that thing in ages.