As a limey myself (someone from the UK), I soon realized that - just as in the UK - Americans have a lot of different accents. But why? I recall seeing something on PBS years ago about it, and it was all insanely complicated.
If you want something a little easier to follow, "Today I Found Out" has a more readable article here.
An accent is “a manner of pronunciation peculiar to a particular individual, location, or nation.” That’s not to be confused with a dialect, which is a specific form of a language that has its own unique lexicon (words), grammatical structures, and phonology (a fancy word for accent). So an accent can be a part of a dialect, but not vice versa.