What is spam?

According to the Collins Dictionary: "In computing, to spam people or organizations means to send unwanted e-mails to a large number of them, usually as advertising (verb, also a noun)".

Historically, SPAM (spiced ham) is a 1936 -created trademark of the American Hormel Food, which crossed the Atlantic to Great Britain in 1941 as a symbol of low-end abundance (unlike canned beef, spam was not rationed during World War II).

In 1970, the Monty Python SPAM sketch coined the term as a symbol of all-invasive presence - there is nothing but spam on the menu of the café Terry Jones works in: "spam, spam, spam and a lot of spam".

What are the different kind of spam?

Here are some possible kinds of spam you may encounter:

Plain advertising spam (UCE/UBE)

Generally promoting a product or a service. Such junk e-mail accounts for the best part of unwanted e-mail. Plain spam is usually related to health, entertainment, finance, although occasional prejudicing, offensive or otherwise controversial content cannot be ruled out (e.g. pornography, politics, religion, etc.).
We can differentiate UCE (Unsolicited Commercial Email) and UBE (Unsolicited Bulk Email). UCE are dispatched by companies which don't respect laws refering to emailing, whatever this is intended or not (mass mailings, suspicious collecting, absent or wrong unregister link...). UBE are much more pernicious: they are sent by professional spammers in unclean ways. These spammeurs invent many techniques in order to prevent antispam solutions and they generally know how to keep anonymity.

Self-spreading viruses

E-mail containing malicious computer code is often sent by the virus itself after activation on a host, using the local user's address book. Subject and text of such messages is often carefully worded to lure the human reader into clicking or some other way activating code within benign -looking attachment.
Once pure technical hackers, the virus creators tend to become more and more linked to spam and downright e-crime players.

Mail bounce notifications

E-mail messages of this kind are barely service messages, notifying the supposed sender of the failure in reaching destination.
This is very much like a telco message "this number is not in service", except that you'll probably get it just because a virus in charge of some friend's PC picked up your address in his/her address book and used it in the sender field of a message to some non-existent destination.

Scam

This type of fraud already had its fax and telephone glory days, yet it has never had as strong a momentum in this day and age of bulk e-mailing.
Whatever the alleged geographical and political background, it basically consists of a strong financial offer for an advance fee (or direct access to a Western bank account).

Phishing

Phishing e-mails are messages that mislead readers into visiting fake websites, usually online banking clones. Once there, giving away genuine identification and passwords to fraudsters is only one click away.
Phishing frauds amounted to 137 million dollars in 2004, from a total of 31,000 attacks (source: TowerGroup).

Where do spam come from?

As Internet in general and e-mail in particular grew to be the everyday tools we now know, advertisers of all kinds got hold of a tremendeous means of communication: e-mail gets almost everywhere within minutes, is subject to easy interaction upon receipt, and most of all costs almost nothing to pile up and send, compared to posted paper.

To make matters worse, spammers take advantage of the inherent slow moves of law and justice to tackle all aspects of electronic junk mail and downright fraud, as well as new users' gullibility and lack of experience.

Spam is sent almost for free... How about receiving it?

In the corporate world, receiving junk e-mail costs a lot: current estimates range between $600 and $1,000 a year per employee (set it times the size of your business and brace for the result).
Of course, such costs only account for human time and IT resources spent to sieve through unwanted e-mail, yet the risk of erasing important e-mail by human error in the process is awesome, though hard to estimate.

Where do spammers get the email addresses from?

Internet providers and routers
Sadly enough, some IT professionals did (and perhaps still do) sell or exchange client data to unauthorised third parties.

Web crawlers
Crawlers are software agents browsing web page after web page, jumping from one link to another and grabbing whatever e-mail address they lay eyes on.

Usenet, mailing lists, chat rooms...
E-mail addresses left on whatever public forum are bound to get captured sooner or later by some specialized software agent.

Hoaxes and pyramidal e-mailing
Some e-mail rumours and charity appeals asking receivers for relaying are known to have contributed to e-mail address collecting.

Viruses
Address book transfer from a PC under virus control to a hacker site is one of the easiest and most silent actions an installed virus can take.

E-mail address harvesting
You probably wouldn't contemplate dialling one hundred telephone numbers from ...5500 upto ...5599 just to get the telco voice answering for some of them "number not in service" and tick them out of your list.
Yet calls through an e-mail channel cost next to nothing and an e-mail server (unless duly instructed) will never frown upon a distant's asking for one hundred thousand combined names, from "abigail.abercrombie@provider.com" to "zack.zoom@provider.com". This is called e-mail harvesting. Such unprotected harvests are ripe all year round.