Stories are the purpley-yellow rings at the top of the main page with people's profile pictures. Or if you go to their profile you'll see the ring around their profile picture. Stuff you post there stays viewable for 24 hours.

Posts are the stuff that appears in the main feed and your profile permanently (unless you remove it manually)

You can also get into the difference between posts are reels but... they've kinda fuzzied that over the years.