Administrators are users who have access to additional abilities on the wiki. These abilities include protecting pages from editing, deleting and restoring pages, blocking users, editing wiki system messages and skins (Interface Administrators), and changing user groups (Bureaucrats). Administrators are users who are highly familiar with the wiki and its policies, functions, and workings. They are not superior to other users and do not have a greater say in discussions; the group exists to make sure the wiki runs smoothly and its content is accurate and presentable. They also handle disputes on the wiki.
You can view a list of all users, optionally filtered by group, at Special:ListUsers.