UBE (Unsolicited Bulk Email) aka SPAM
Most simply explained, spam is junk mail. Most every internet user has encounterd spam.
Most web-users look unkindly upon spam and Internet Connection shares this viewpoint. We have a portion in our User Agreements (CSA & VARSA) detailing our policy regarding network abuse such as spam and the consequences of violating our agreements.
On a more light-hearted note, here's an entertaining article entitled "Death to Spam". It contains some useful information on combatting spam as well as some insight into the mind of the spammer.