Administrators are members who have "sysop rights". Current policy is to appoint new admins when it is felt that the current number of admins is insufficient. Administrators are chosen from contributors who are active and who have been around for a while and are generally known and trusted members of the community.
Other access types
In addition to administrators, there are other types of identified users, listed here in roughly ascending order of power.
Users with ordinary access, including visitors who haven't "signed in," can still do most things, including the most important: editing articles and helping with maintenance tasks.
But only signed-up users can upload files or rename pages
Users with bureaucrat status can turn other users into sysops. Bureaucrats are created by other bureaucrats on projects where these exist, or by Wikia staff on those who don't yet have one.
The Wikia technical team can make direct changes to the Wikia software and database. These people, by and large, do not carry out administrative functions. The best way to report problems or bugs to them is via Special:Contact.
If you think an administrator has acted improperly against you or another editor, you should express your concerns directly to the administrator responsible. Try to come to a resolution in an orderly and civil manner.