What is a domain name?
All sites on the web are identified by one or more domain names. The domain name for Museums Sites, for example, is museumssites.com. It is perfectly acceptable to use either museumssites.com as well as www.museumssites.com.
As part of your Museums Sites package, your new website will be configured as username.museumssites.com: this is called a subdomain. While it is acceptable to use a subdomain and you will need to use it to create your website (create content and upload text and images), once you decide to go live you may want to use your own fully qualified domain name (such as www.mydomain.com).
When you point your fully qualified domain name to your Museums Sites subdomain, your site will go live.
This is really simple to do and there are other articles with detailed instructions in this section, but if you are in trouble do not hesitate to get in touch.
Please note that fully qualified domains are not available in the free edition of Museums Sites.