Spamming is the practise of sending a message to a huge number of email addresses or Usenet Newsgroups. It's usually done for commercial reasons or to scam people into parting with their money. The result is that people are plagued with junkmail which costs them time and money (since the time spent downloading this rubbish increases your phone bill). The list of people that the spammer sends to is generally automatically generated by scanning Usenet Newsgroups, WWW search engines and so on.
There are a variety of anti-spam techniques and organisations in existence. Some aim to help you to stop individual spammers, some are fighting to stop spam via the law courts and others lead campaigns targetted at forcing Internet Service Providers to put a stop to it.
The best anti-spam service I've found is The Network Abuse Clearing House. You send them the spammed email; they do the rest. I would encourage you to subscribe. Start by sending a mail to firstname.lastname@example.org.
Here are some other Anti-spam links:
Fight Spam on the Internet!
CAUCE - Coalition Against Unsolicited Commercial Email. This is the major group trying to force the US government to pass laws against spamming.
James Masters Home Page
james at masters.me.uk