Different users have access to different functions of the site. While anyone can do most things on the site, including reading and editing, administrators can access a few additional functions.
Bureaucrats are administrators who, in addition to the admin functions, have the ability to change the user rights for administrators and rollbacks and promote users to bureaucrat.
Administrators are entrusted with the ability to block and ban users as well as edit and delete comments/pages. They can also change the protection settings of pages and promote/demote chat moderators.
How do I use administrator powers?
See Help:Administrators' how-to guide for a guide on using admin functions.