Gmail or Google Mail became available as a beta release in 2004 and was launched to the public in 2007. Gmail is a product provided by Google to service web mail for its users. Initially it was launched as an invitation-only release. Though Gmail is considered a free service, Google incorporates advertising within its interface in order to raise revenue.
image source: http://en.wikipedia.org/wiki/File:Gmail-01-01-09.png
Gmail also offers its users with Google Voice and Video chat. Further details are available in this video:
In 2004, Loren Baker reported that Google came under fire with "complaints against Google's Gmail and its use of AdWords Contextual Advertising to show text ads targeted to actual email content" (Baker, 2004).
Baker, L. (2004). Google May Change Gmail Advertising Model Due to Complaints. Retrieved July 25, from http://www.searchenginejournal.com/google-may-change-gmail-advertising-model-due-to-complaints/450/