Spam email is a form of commercial advertisement trough email which is a very cost-effective medium for the sender. With the amounts of spam emails that are sent, if a small fraction of the receivers purchase the advertised product, the spammers are making profits and the spam is given more strength.
Spammers get a bunch of email addresses through publicly accessible sources or through programs that collect addresses on the web.
At the present, it is assumed that an average of 95% of emails that are sent globally are believed to be spam, making this phenomenon an ever growing one.