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.
Some apparently unsolicited e-mail is, in fact, e-mail people agreed to receive when they registered with a site and checked a box agreeing to receive postings about particular products or interests. This is known as both opt-in e-mail and permission-based e-mail.