Spam is more or less defined as any e-mail you don't want and didn't sign up for. It is usually advertising something or promoting some kind of scam. Other terms for it include unsolicited bulk e-mail (UBE) and unsolicited commercial e-mail (UCE).
Spam is widely recognized as a serious problem facing ISPs and internet users, since it eats up bandwidth, defrauds people, and fills up people's inboxes with junk. It has the potential to become much more of a problem than junk snail mail or telemarketing (phone spam) because it is so unbelievably cheap to send spam e-mail rather than calling you or mailing you a piece of paper. Some people literally are forced to throw away their e-mail addresses because they receive so much unwanted spam.
If e-mail servers are continuously overwhelmed with spam, it could eventually become a threat to the very existence of e-mail as a communications medium.