Most of the website can be accessed by adding a www to the front of domain name, and without. For example, you can visit technicalguy.org or www.technicalguy.org to visit the same website.
Technically speaking, website address is used to point to an IP. To direct visitors typing the URL to the correct web server located on different IP. You can view both URL as two different address, pointing to same IP.
In most case, you should define a master one, which I suggest to be the one with www. Then, point the one without to the main one using redirect 301, which is much friendly for search engines.