Unwanted, unsolicited email.
The definition of email spam is purposely vague because everybody has his or her own definition. As it currently stands, email spam is in the eye of the beholder. Email spam usually pertains to unsolicited commercial messages sent in bulk by people you don’t know–although there are exceptions to this rule.
Some spammers will argue that email spam is not any different than traditional junk mail, but there is one undeniable difference… cost. The cost of sending junk mail through conventional means is very real, including postage costs paid by the sender. The cost of sending email spam is very small, and most of the costs are paid by people other than the sender.
The lack of significant barriers (cost) to entry (sending) is often cited as a key problem with email spam. With traditional junk mail, a direct marketer must ensure that the offer & targeting are good enough to more than offset the costs of mailing. With email spam, anything goes, often in the form of shady deals and outright scams.
While the monetary costs of emailing are small, there are other non-monetary costs that junk emailers face; the risk of getting kicked off of their ISP or webhost, plus the risk of tarnishing their name within the legitimate marketing community.
see also: Email Marketing Tools