re: Please explain to me what an atheist is.
"atheist" literally means "no gods." Generally, it's used to mean someone who doesn't believe in any of the gods of major religions. An "agnostic" is like a political "undecided." It just means "leave me alone, I don't want to talk about it."
Beyond that, it gets a bit fuzzy. There are many people who don't believe in God or Allah or whoever, but are superstitious as hell in other areas. Is someone who believes in something supernatural but not gods an atheist?
Consistent atheists like Asimov or Sagan were hard over negative on all supernatural ideas. IMO, someone who claims to be a rational atheist but sorts his house out according to feng shui is scraping the siderail a bit.