"Spam" can be defined in many ways, but many people define spam as all unsolicited e-mail that is sent in "bulk" to many addresses.  Many people also think of spam as the on-line equivalent of "junk mail" that you likely receive in your postal mailbox every day.  Although some definitions of spam focus on the unwanted e-mail that is commercial in nature, many people also think of spam more broadly to include such items as e-mail scams (such as those from supposed relatives of expired Nigerian officials) and those annoying chain letters that promise "good luck" for forwarding the chain-letter to many other people.