Spam is junk mail. Spam is "make money fast" and "free porn." Spam is unwanted e-mail, usually of a commercial nature, sent out in bulk. Spam is Unsolicited Bulk E-mail (UBE). The opposite of Spam is \["Ham"\]. 

The word SPAM is actually a registered trademark of \[http://www.hormel.com/ Hormel Foods Corporation\]. Their \[http://www.spam.com/ci/ci_in.htm trademark guidelines\] say:
  We do not object to use of this slang term to describe UCE, although we do object to the use of the word "spam" as a trademark and to the use of our product image in association with that term.
  Also, if the term is to be used, it should be used in all lower-case letters to distinguish it from our trademark SPAM, which should be used with all uppercase letters. We hope that Spam with an uppercase letter S (as in [SpamAssassin]) is ok, too :)