On UNIX this special user is known as superuser or root (not to be confused with the root directory). Superuser can override file security and do almost anything she wants on the system (she cannot see your password, since it is encrypted, but she can change it). In fact, any user with a userid of 0 is a superuser. Naturally, such users should always have a password.
It is not good practice for the system administrator to always logon
as superuser. It is too easy to make a trivial mistake and damage the system,
perhaps by rm * in an important directory. Instead, logon as
a regular user, then switch to superuser with the su command when
you need it.
Root is Also the Start of the Directory
In a Hierarchical
File System, one directory is the root or start of the tree. Other
directories hang off root and they in turn can have subdirectories.
On UNIX and POSIX, root
is specified as a forward slash "/". On DOS, root is specified as
a backward slash "\". This meaning of root should not be confused
with the alternate meaning of root as the UNIX system manager (that
is, superuser).