In its original meaning SPAM (all uppercase) is the proprietary name for a type of processed, canned pink luncheon meat from Hormel, primarly made from pork. The name is a contraction of "SPiced hAM". It was very popular in England during World War II, when fresh meat was in short supply.  Most Americans intuitively, viscerally associate "SPAM" with "no nutritive or aesthetic value," though it is still relatively popular (especially in Hawaii) and can be found in almost any grocery store. SPAM has its own newsgroup, alt.spam.
However, spam has now acquired a completely different meaning, as electronic junk mail or junk newsgroup postings. There is some debate about the source of the term, but the generally accepted version is that in the days of internet bulletin boards, unscrupulous users would create post after post of "SPAM SPAM" etc in order to push other people's topics off the bottom of the board before posting their own unsolicited advertising message. The term "SPAM" was used due to the popularity of a particular episode of Monty Python's Flying Circus, set in a café, which served dishes such as "Egg, spam, chips, spam, beans and spam", and which ended with a song which went: "Spam spam spam spam, spam spam spam spam, lovely spam, wonderful spam…" Like the song, spam is an endless repetition of worthless text. 
Another school of thought maintains that it comes from the computer group lab at the University of Southern California who gave it the name because it has many of the same characteristics as the luncheon meat Spam:
- Nobody wants it or ever asks for it.
- No one ever eats it; it is the first item to be pushed to the side when eating the entree.
- Sometimes it is actually tasty, like 1% (or less) of junk mail that is really useful to some people.
A "spam" posting on a wiki often includes worthless text disguised as educational information accompanied by a link to an external website which is selling something; it is an attempt at free advertising.
In video games, spamming refers to repeatedly using one technique or move.
There are laws in many nations prohibiting or restricting the production of email spam. In the United States, for example, spammers are supposed to adhere to restrictions under the CAN-SPAM Act of 2003 as well as any relevant state laws. However, many spam operations are part of organized crime involving otherwise illegal activities such as illegal sale of prescription medications, counterfeit products, online gambling, phishing scams, and malware. Many spam messages are produced by botnets consisting of infected machines around world.
Hormel Foods have expressed that they are not fussed about the usage of the term "spam" to denote unsolicited bulk email, provided that the distinction is made between the uppercase and lowercase forms of the word.
- Official SPAM Website
- The Net Abuse FAQ. 2.1) What is Spam?
- What is spam?
- SPAM brand and the internet