google is your friend:
http://www.iep.utm.edu/n/nihilism.htm
seriously, though, nihilism seems to me to be pretty extreme: values mean nothing, you exist for no reason, all is corrosion and destruction. you said it, you believe NOTHING, and nothing has value.
what's most interesting in my mind about nihilism is its total rejection of most peoples' innate desires of belief and optimism.