The "Surprise" Symphony. It comes from the original German nickname "Paukenschlag," meaning bass base drum impact. Haydn was well aware that more often than not, members of the audience would fall asleep during his performances. Probably out of a "I'll show you!," kind of place, Haydn created this symphony. After a lovely first movement, he created a simple innocuous melody in the second movement followed by a large "impact" to wake those who fell asleep.
by Aaron Green, About.com Guide