Most of them are faces used to denote varying levels of seriousness.
  1. 😐
    The main sarcasm face.
  2. 😑
    My "what you said didn't make much sense and/or was stupid but ok." face.
  3. 😏
    Smug face.
  4. 🙃
    When I have an idea or do something fundamentally eccentric/moronic, that's the face.
  5. 🙏
    The best way to get something from someone is to use this emoji coupled with, "plz, plz, plz, plz, plz, plz, plz, plz, plz, plz, plz, plz, plz, plz..."