From Joomla! Documentation
As a website administrator you might be concerned about the visual appearance of a website but will pass responsibility for making changes and improvements to a web designer. Similarly, when changes to functionality are required you will contract a web developer to do the necessary work. As a website administrator you are responsible for the day-to-day operation and maintenance of the website. Activities might include ensuring that the site is properly backed up; managing user access; possibly installing extensions. You will also have responsibility for maintaining the security of the website.
Joomla! Administrator's Manual
Note from the Doc Team: The intention is that the list of topics you see below should be task-oriented and not a "feature list" for Joomla! website administration. These items should address real and common activities that an Administrator will need to perform.