Wiki discusses it here: http://en.wikipedia.org/wiki/Crank_(person)

In particular, I'd like to point out

"Cranks overestimate their own knowledge and ability, and underestimate that of acknowledged experts."

"seriously misunderstand the mainstream opinion to which they believe that they are objecting,"

"stress that they have been working out their ideas for many decades, and claim that this fact alone entails that their belief cannot be dismissed as resting upon some simple error,"