Internet payment help!?

I got a letter from my internet provider (wireless. don't know if that matters but...) saying that we had went over the gigabyte limit. I really don't understand. I know gigabytes are some computer memory measurement. But I don't get how that has to do with internet usage! If every internet provider charges the same way then someone help me, because I want to download an album and my aunt won't let me because she thinks it will count as some of those gigabytes. I'm 13 and not american so if there are any misspellings I'm sorry :)