A Yognaut, or Yognaught, is anyone who is a fan of the Yogscast.
Every Yognau(gh)t's name is Dave!, and is instructed to do the special Yognau(gh)t salute if they ever meet another Yognau(gh)t in public. The salute is done by claiming that you are both Dave! Yognau(gh)t, and that you have the balls (see Dave! Yognaut).
In "YoGPod 26: Your Grandad Was A Bee?", The Yogscast instructed people to call themselves Yognauts if they prefer Lewis, and Yognaughts if they prefer Simon. The reason being that Lewis thought of Yognauts as being like astronauts, while Simon first used Yognaught because, as he said, it's "Naught, as in nothing".
The reason both are being used is because Simon likes the fact that he is getting someone to spell a word "wrong", even though it is a word they made up.