The only valid characters for a domain name are letters, numbers and a hyphen "-". Other special characters like the period (dot) ".", the underscore "_", or the exclamation mark "!" are not permitted.
http://www.example.com is a uniform resource locator (URL) used by web browsers to find documents and resources on the web. The "www", however, is not part of a domain name (though a domain name is necessarily part of a URL). It should never be used on the domain name registration. For more details visit the InterNIC site.