www.example.com is a subdomain of example.com. Just like foo.example.com or bar.example.com. Many domains direct their www subdomain to their base domain and vice versa using DNS, but they can resolve to different IP addresses.

www was just a convention for notating that the server was for a website on the “world wide web”. As opposed to mail.example.com being for an email server or ftp.example.com being for a File Transfer Protocol server.