Usually it is an author. Mark Twain was actually Samuel Clemens.
Mary Ellen Evans took on the name of George Eliot (look up MiddleMarch).
Sometimes it is narrowed more as in The Red Baron -- you could think snoopy's arch enemy. The Red Baron was actually Manfred Von Richthofen. His enemies cared nothing about the distinction. All they knew was that when he was flying, he was dangerous and he needed to be avoided if possible.
So a pseudonym is a name taken on to disguise a real name.
