Once upon a time, SPAM was an animal by-product that came in a can and was fodder for many Monty Python sketches, but since the world-wide adoption of the internet as an integral part of daily life, Spam has become synonymous with what is wrong with the internet. Spam, in general terms, is an email or other forms of unsolicited advertising. Spam is very easy to spread throughout the internet, and works on the principle that if you send out thousands, or hundreds of thousands of unsolicited advertisements, scams, or other questionable methods of making money, that you only need a very small percentage of people to be fooled and you will make lots of money.

Common spam these days comes from online gambling sites and those trying to sell drugs for "male enhancement." Lately, web logs, or blogs, as we call them, have been targeted by spammers to try to increase their site ratings in the search engines. Spammers use various methods to distribute their electronic junk mail, and employ bots, or computer programs to quickly and easily send email or comments to millions of addresses and IPs all over the world.

Spammers can be difficult to track down as they often hijack peoples' email and IP addresses. When this happens, it may appear a friend sent you the spam, but in fact, the spammer's bot grabbed your friend's email address and used it to hide the true source of the spam. WordPress developers and community members are constantly working on more and better ways to combat these annoying spammers as they clog the internet with their garbage. You can help by offering your talents, ideas, suggestions, or just by being vigilant and installing any of the currently-available spam combating tools.

Reference http://codex.wordpress.org/Glossary