SPAM is unsolicited "junk" e-mail sent to large numbers of people to promote products or services. If people would not buy the products or services advertised in spam, the use of spam would die away. Unfortunately sufficient people respond to the unsolicited junk e-mail to make it profitable for the original senders.
Spam has become so successful that it is now being propogated by viruses that spread throughout the internet. Many modern viruses send spam from every infected computer. Using viruses to send spam has significantly increased the volume of spam being circulated over the internet.