In a Google account (sync), what is the difference between Contacts and People details? For a Google account, you are allowed to sync Contacts and People details. What is the difference between these two?
The best explanation I could find online:


  People Details is a new sync service related to the recent release of
  Google Play Games. It was pushed to most (if not all) android devices
  recently. Its purpose is to tie you together across all Google Play
  services (the separate services available from the Google Play
  screen). The idea is to maintain a uniform and unique "identity" for
  you in all related Google Games so that data from one can potentially
  be used to "enhance" the user experience in another


The contact sync is for the usual contacts that a user saves directly on your phone.