An atheist is someone who is sure that there is no G-d (or any kind of god, lhavdil). An agnostic is someone who isn’t sure if there is a G-d.
That’s why some say it’s impossible to be an atheist – because how can you be sure that G-d doesn’t exist?
Here’s a joke: Did you hear what happened to the dyslexic insomniac agnostic?