UGAL belongs to the family of Content Management Systems, commonly refered as "CMS". From its wikipedia definition, a CMS is a "computer application used to create, edit, manage, search and publish various kinds of digital media and electronic text". Using a CMS for managing your website has many advantage versus the traditionial approach of creating static HTML files and uploading them to a web server.
With a CMS the data for your website is stored in a database, not in static HTML files. When a page is requested, the database is queried and the data that is retrieved is formatted on the fly into HTML that the browser can display. The formatting includes "decorating" the data with your website design and layout. By separating the content of the page from its surroundings, the dynamic approach allows for a lot of flexibility that is not possible with static websites. A few examples:
A CMS brings you more than just a dynamic website. It brings you a complete application (program) to manage every aspect of your website. In the case of UGAL, the application has the extra benefit of being web-hosted.
While your visitors see what they believe is a "normal" website, you and your website administrators have access to a suite of applications to:
CMSes are available in all shape, colors and prices. Instead of playing the game of "the most functionalities", UGAL wins the game of implementing functionalities that are actually useful in a interface that remains very easy to use. One example is the Onsite Editing feature, that allows to edit your website while you browse it, not from some obscure dashboard.