On a computer or computer network, an administrator is a person that has a user account or is part of a user security group that has complete access to the computer system. This means that they are able to create, delete and modify files, folders and settings on that computer or computer network. This is different from other user accounts that only certain permissions and levels of access are granted, limiting what those users can do. An administrator account is primarily used to make changes to the entire system such as creating or deleting user accounts on the computer or computer network, creating account passwords for other user accounts, and changing account names, pictures, passwords and types.

If a user has administrator rights, it simply means that they have been given unrestricted access to do anything that they wish on the computer system. This is in contrast to administrative rights that are granted to other users by the administrators to allow them to create, delete and modify items and settings. It is very important to know the user name and password for the administrator account on your computer. If you do not know it, you will be unable to install software, change settings or fix problems.

In Windows, the account named Administrator has all of the possible rights on the computer system. Anyone that has an account in the Administrator local security group also has these rights. Normal users of the computer also have some minor administrator rights that will allow them to modify anything in their home directories, but any changes that would affect the entire computer have been restricted.

Computer administrators cannot change the administrator account to one of a less privileges unless there is at least one other administrator on the computer system. This will ensure that an administrator always exists for that computer.

Ideally the administrator account on a Windows computer should only be used to install, repair, backup or upgrade the operating system, install service packs, configure operating system parameters or take ownership of files that have become inaccessible.

On UNIX based operating systems, there is usually one account for the root or superuser. This account has full access to all files and directories and many of the low level tasks in a UNIX based operating system must be run as the root user.

Because there is so much unrestricted access to the computer for the root user, computer administrators in this environment often will only log into a regular account and then use the sudo command with their root password, should they need access to perform a certain task.

Normal users on a UNIX computer cannot use the sudo command, but they do have the privileges required to install some software and customize their environment. Each user on a UNIX computer also has a home directory that they have full control of.