The superuser is the term for the system administrator on many computer operating systems.

Unix

In Unix-style computer operating systems, root is the conventional name of the user who has all rights or permissions in all modes (single- or multi-user). Alternative names include baron in BeOS, toor ("root" backwards) and falken in NetBSD, and avatar on some Unix variants.

The user root can do many things an ordinary user cannot, such as changing the ownership of files and binding to ports numbered below 1024. Generally, it is not good practice for anyone to use root as their normal user account, because simple typographical errors can cause major damage to the system. It is recommended to create a normal user account instead and then use the su command to switch when necessary. Some use the sudo utility instead, which allows a measure of graduated access.

Software defects which allow a user to "gain root" (to execute with superuser privileges code supplied by that user) are a major security issue, and the fixing of such software is a major part of maintaining a secure system. One common way of gaining root is to cause a buffer overflow in a program already running with superuser privileges.

If a person "has root access", it means that they are able to act as the administrator of that computer.

The name Charlie Root is often associated with the root account, named after the baseball player of the same name.

DOS and Windows 9x/Me

DOS and the DOS-based versions of Microsoft Windows (i.e. Windows 3.x, Windows 95, Windows 98, and Windows Me), are not multi-user operating systems, and there is no distinguished superuser account. Effectively any user of the system has administrator privileges. Separation of administrative privileges from normal user privileges makes an operating system more resistant to viruses and other malicious software, and the lack of this separation in these operating systems has been cited as one major reason why these operating systems are more insecure.

Windows NT

In Windows NT and later systems derived from it (Windows 2000, Windows XP, Windows Server 2003 and Windows Longhorn), there may or may not be a superuser. By default, there is a superuser named Administrator, although the situation is not an exact analogue of the situation with Unix.

The Administrator account does not have all of the privileges of the Unix root account, some functions of the Unix superuser being instead the domain of the Local System account in Windows NT.