Unsolicited bulk email is often referred to as spam, an unfortunate use of the Hormel Corporation's luncheon meat of the same name.
The key word here is unsolicited: spam, like junk mail, is not something that you asked for. It appears in your inbox (often multiple times), taking your attention away from relevant work-related and personal email. Spam costs millions of dollars a year in lost time, due to slower bandwidth, attempts to filter, and frustration.
You may have noticed a lot of pointless email in your inbox. These messages tout schemes to get rich quick, or pills to look younger, or deals on toner cartridges -- or offers that are even less savory. Who are these people and how did they get your address? For the unscrupulous senders of these messages, email is a form of cheap advertising, a venue for scamming people, or a way to shock or annoy.
This type of email, sometimes called spam or unsolicited commercial (UCE) or bulk (UBE) email, is becoming more common, and more annoying. While the recent state of affairs with regard to spam has not been bright, there are legitimate organizations working to combat it through proposed legislation, spam reporting, and better filtering options.
MIT offers Spam Filtering on its mail servers: