www.example.com is the hostname of a webserver. www is the traditional hostname given to webservers, the same way ftp is the traditional name given to FTP servers. This is just a convention. BBC News webserver is news.bbc.co.uk for instance.
After all isnt it all the web?
Email isn't part of the 'web'. Nor is FTP, USENET, P2P... Websites arn't the only thing using the Domain Name System. This is why many people got very upset with Verisign's Sitefinder.