In the online world, the term spam means unsolicited bulk email, an online version of junk mail that you get in your snail mail mailbox.
Two of the biggest benefits of email are that it is virtually instant and free. Because of this, it is a great advertising medium. There are two ways to use email for advertising. The first way is to ask permission of users to send them email. This is what legitimate companies do. On the website of businesses and publications, such as www.CNN.com, the New York Times, and others, you will be able to opt-in to receive regular emails with news and updates. The second way of sending email is spam. With spam, advertisers and scammers send emails without asking for permission.
Spam is an extremely popular way of advertising and scamming because of its low costs. With direct mail, advertisers need to pay for stamps. With spam, they can send millions of emails for a very little cost. As you may expect, 
people that use an ethical way of advertising, are likely to advertise products such as get rich quick ventures, unapproved health supplements and so on. Many of such messages also offer worthless stocks.
Spam is a really big problem on the Internet and spammers are sending millions of pieces of spam a day. Spam doesn’t even have to have a commercial nature. It can also be religious or political. The nature of emails doesn’t really matter. If you didn’t ask for it and it is not an individual email, then it is spam. You may not even realize it, but spam costs you a lot of wasted time and computer resources.