All www is is a naming convention inherited from hostname/domain name setups and procedures that existed when the Internet existed, but the “World Wide Web” (aka: www) was a new thing.
So—for example—at one point in the early 1990s a company named example.com could have had a simple Internet presence for the use of users using Telnet. So a user on example.com could Telnet into example.com by running a command like telnet example.com. But if they used a web browser like Mosaic or Lynx at the time, they would not get any content from example.com.
But now example.com set up a web server and needs a new home for it. Since the world wide web was not as common back then as it is now, simply going to http://example.com would have been confusing. So instead the hostname convention of www.example.com was devised. This naming convention followed the logic of FTP servers being on ftp.example.com and such.
So flash forward to nowadays, 99% of most people online connect to a hostname/domain name purely for some type of web server access. So now accepting HTTP connections on http://example.com would make sense in addition to http://www.example.com. In many cases, systems administrators setup redirects on the non www hostname so http://example.com automatically gets redirected to http://www.example.com.
So in short, in most cases http://example.com and http://www.example.com should give you the same content. The two naming formats exist to respect historical conventions/standards in hostname/domain name practices. And many times administrators just redirect requests to http://example.com to http://www.example.com to make their lives easier.