It's hard to explain. There's technically several in Pennsylvania alone, due to various mountains within the greater Appalachian range separating people in the days before cars, and a mixture of nationalities settling here (English, German, Polish, French, the list goes on). It's not like that in every state though. New York, Boston, and Southern are the main ones people tend to think of though.
I'd actually suggest you search on YouTube for various accents; I know there's something called an Accent Tag that you can start with. The generic American accent that newscasters attempt to use is called either "General American" or "Standard American" if you're looking for examples.