Spam, in general terms, refers to the sending of copies of a message to multiple people. The number of copies could be hundreds of thousands, or it could be only a few, or even just one single message. A common type of spam is that sent to Usenet newsgroups. The culprits like this method, because by posting a single message to one or more newsgroups, they could potentially reach millions of people. This term applies to the sending of marketing-type emails to people who have not agreed to receive them.
Most spam consists of advertisements for dodgy products or services or get rich quick schemes.
The main qualifying factor of all spam, however, is that it is unsolicited . Sending a message to multiple people, when they have agreed to receive it , does not constitute spam. Spam is very cheap to send. In other words the receiver pays. It's as if you had to pay for the printing and delivery costs of all the junk mail you get in your letterbox.