Spam, of course, is a canned food product. Some people claim that it's tasty, and some believe it contains meat. Click on the Spam logo at the top of this page if you want to see all kinds of neat Spam merchandise!
There are several definitions for spam. Originally, it referred to the practice of posting separate copies of the same article to multiple newsgroups. It had nothing to do with the content of the article; you could post an article in several newsgroups in which it was appropriate, and it would still be spam if you posted separate copies rather than crossposting.
Recently, there have been many people and companies who have taken the idea of junk faxes (send someone an unsolicited fax advertising your products or services, and expect them to pay for the consumables used by their fax machine while receiving it) and moved it to Usenet news postings and to email. This is a very objectionable practice to most people, partly due to the inconvenience and partly because the recipient pays to receive unsolicited matter which they probably didn't want. This activity is also known as spam.
So why's it called spam? Monty Python did a routine about Spam, and part of the routine was an annoying, repetitious chorus of "Spam, spam, spam, spam!" So an annoying, repetitious stream of unwanted messages is called spam.
Most spammers gather addresses from one of four sources:
So one way to avoid spam is to become a hermit, and stop contributing to the discussions in newsgroups. That's not a great idea, though, because you're letting the spammers restrict what you can do. Another idea is to put fake email addresses in the headers of your articles, and put your real address somewhere within the body, since many spammers only gather addresses from headers. I'm not fond of this method, either, but it works pretty well for some people (though it will cut down on the number of people who will bother to send you legitimate email; the more roadblocks you put up, the fewer people will take the time to navigate them).
A better way is to complain (politely) to the postmasters or abuse addresses at sites which send spam. All sites on the Internet are required to accept mail addressed to postmaster@their.domain.name; however, some do not. Many, particularly larger ISPs, also have specific abuse reporting addresses (usually abuse@their.domain.name). Most ISPs have formal policies regarding net abuse and, should they determine that one of their users is abusing the net, will take action ranging from user education to account deletion.
A great tool for fighting spam is abuse.net.
One other thing I should note is that most spam (at least, most of the spam that ends up in my mailbox) has faked headers and fake reply addresses. Usually, some of the header lines are legit, and some are fake. Clearly, spammers know they're doing something wrong, or else they wouldn't try to hide their identities.