google.com is a domain name, this points to a nameserver.
www. google.com is an A record, commonly known as a sub domain, which points to an IP address, usually of a web server.
google.com can also be an A record, which points to an IP address, usually of a web server.
Traditionally websites have been accessed by using www. domainname.com, and generally advertised that way. Typing www. or not does not guarantee you access to the website you are attempting to, though most properly configured dns records will allow either to be acceptable. If the A record for domain or the subdomain (www) is not present, then you will be greeted with an error message.
It is recommended to accept requests from both, but forward one to the other. For example I ensure all my websites work with or without the www, and if they type it without the www, they are forwarded to the www. This reinforces the fact that it is a website, as domains can be used for many purposes.