Every website on the internet has a domain name. This is sometimes also called a web address. A web address is similar to a physical address or phone number, but for your website. It is what allows people to find your unique website on the big world wide web and distinguish your site between all the other sites that are out there.
With Yola, you have the option to use a free subdomain. If you choose this option your web address will end in .yolasite.com and it will be a subdomain of our domain.
The other option is to buy your own custom (or "top-level") domain. It would end in .com (or one of the other domain extensions available). Custom domains are shorter and easier to remember and type - so more people will be able to find your site. They also make your website appear more professional and your rankings with the search engines (such as Google) will be better.
For more information on buying a domain from Yola, see: Buying a Domain from Yola Tutorial.