A pseudoynm is a fake or alternative name given to someone;
- a) to disguise their real identity
- b) because it sounds cooler than their original name.
Vladimir Lenin's real name was Vladimir Illyich Ulyanov, but no one would remember it, so he changed it to Lenin.
David Cameron's real name is David Dirty Political Opportunist-eron. But he changed it because he thought everyone would think him a "dirty political opportunist" for some reason.