[Q] Difference between at&t HTC One and HTC One unlocked?

Are there any difference between an AT&T HTC One and an unlocked HTC One (bought directly from the HTC website)? Also the HTC One Google Play edition, what's the differences?

I have never used an android phone before so this will be my first one. If there are differences, which one would be "better" (for a noob like me)?

Thanks