Spam is flooding the Internet with many copies of the same message, in an attempt to force the message on people who would not otherwise choose to receive it.

Most spam is commercial advertising, often for dubious products, get-rich-quick schemes, or quasi-legal services. Spam costs the sender very little to send -- most of the costs are paid for by the recipient or the carriers rather than by the sender. Email spam targets individual users with direct mail messages. Email spam lists are often created by scanning Usenet postings, stealing Internet mailing lists, or searching the Web for addresses. Email spams typically cost users money out-of-pocket to receive. Many people - anyone with measured phone service - read or receive their mail while the meter is running, so to speak. Spam costs them additional money. On top of that, it costs money for ISPs and online services to transmit spam, and these costs are transmitted directly to subscribers. Spammers use special programs that extract email addresses from Web sites and Usenet postings. To avoid ending on a spammer's mailing list when you post to a Web forum or a newsgroup, you can obscure your email address by inserting something obvious into it. 

Never reply to spam messages, even when they entice you to reply to "remove" you from their mailing lists. Often the instructions are either bogus, or a way to collect more addresses. Replying confirms to the spammers that your e-mail address is active, and you may receive even more junk mail.