The history of the term Spam starts with episode 25 of Monty Python's Flying Circus, and the Spam Sketch in which a group of Vikings sing the word Spam over and over, so loudly that no-one else can communicate
After a short detour through MUDs, where it was used to describe someone shouting the same thing repeatedly, stopping any interaction between other people, it came to usenet.
On usenet the term spam is used to refer to the practice of posting an article, often an advert for a dubious porn site or a ponzi scam, many, many times. This might be many times to one group or more usually it'll be posted to a lot of groups.
More precise terms for usenet spam are Excessive Crossposting (ECP) - crossposting the same article to many groups - or Excessive Multiposting (EMP) - posting substantively the same article many times, to each group individually
Most usenet spamming is a mix of ECP and EMP - the spam will be crossposted to many groups, many times
Usenet spam is automatically detected and cancelled by cancelbots, but because of the way usenet propagates some percentage of the spam will make it's way through to readers before the cancels catch them
Many sections of usenet have been turned into wastelands - whole hierarchies have been so deluged by spam that it's impossible to use them. The members of the groups have left, and there's nothing there but spam.
Around 80% of usenet traffic is caused by spam
Killfiles. All real newsreaders have a killfile - this is a way of filtering out posts based on subject, poster or a number of other things. These are good for filtering out the background noise and occasional spam in an otherwise good newsgroup. Web browsers that claim to be newsreaders seldom have usable killfile features (Under Win95/NT Forte Agent and Anawave Gravity are two commonly used newsreaders)
NoCeM is a way to allow someone else you trust to filter out articles for you. At the time of writing no Windows client software is available, but it is possible for your news adminstrator to use NoCeMs to delete spam from the server by installing NoCeM on spool software
Server filtering. It's possible to detect and delete the huge majority of usenet spam using server filters. Spam Hippo is a filter from Zippo news, which they make freely available to ISPs who wish to run it
Shunning. At any one time the majority of spam comes from a few sites. By shunning them, refusing all articles from them the amount of spam drops
Hunt the perpetrators down. Complain to their upstream provider. Repeatedly. Get them shut down. Get the servers, web pages and email addresses of the advertisers yanked. Report the illegal schemes to the local police.
The 'correct' term for email spam is Unsolicited Bulk Email (UBE), though you'll see the term Unsolicited Commercial Email (UCE) used more often.
Spam is popular amongst the scammers, the multi-level marketing crowd and the porn pushers because it costs them so little to send. This is because they send it by stealing the resources of others.
There are three main flavours of email spam
Filter them out or bounce them back. If you receive email via a unix system you may be able to run procmail filters. If not you can use the spam filter capabilities of your mail reader, or one of the many third party filtering tools
Hunt the perpetrators down. Complain to their upstream provider. Repeatedly. Get them shut down. Get the servers, web pages and email addresses of the advertisers yanked. Invoice them for your time, sue them in a small claims court when they don't pay up. Report the illegal schemes to the local police.
Tracing the perpetrators using spamware, or the spamhauses requires some familiarity with email headers - see the tutorials for more information on those