Aside from www.Google.com (in which a thousand results would enlighten you), I'll give it a shot!

[QUOTE]Source - WIKIPEDIA - http://en.wikipedia.org/wiki/E-mail_spam

Spam by e-mail is one type of spam that involves sending identical or nearly identical messages to thousands (or millions) of recipients. Addresses of recipients are often harvested from Usenet postings or web pages, obtained from databases, or simply guessed by using common names and domains. By definition, spam is sent without the permission of the recipients.

Bulk e-mail or unsolicited bulk email (UBE) are sometimes used as more precise or less slang-like expressions for spam.


Other Related Terms

Source - www.iptv.org/digital/dictionary_internet.cfm

Spam is unsolicited e-mail on the Internet. From the sender's point-of-view, it's a form of bulk mail, often to a list culled from subscribers to a Usenet discussion group or obtained by companies that specialize in creating e-mail distribution lists. To the receiver, it usually seems like junk e-mail. In general, it's not considered good netiquette to send spam. It's generally equivalent to unsolicited phone marketing calls except that the user pays for part of the message since everyone shares the cost of maintaining the Internet.


Source - www.ruf.com/mglossary.html

Bulkmail (as per the meaning of US Postal Service) - Second, third, and fourth-class mail serviced on a non-preferential basis by the US Postal Service.
* (my silly editor ctrl+c ctrl+v s*cks!)
My shot, SPAM is the *geeks generic name for unwanted unsolicited mails, (READ IT - SHOULD-NOT-REALLY-BE-IMPORTANT MAILS MOST OF THE TIME ANNOYING!) while bulk mail could be one of it's extension name, also, you may call it junk mail if you like.
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Hope that helps!

Yo!