From LQWiki

In Linux and UNIX, the root account is the name for the system administrator, which has full permissions to perform any task available on the system. The root account has a user ID of 0.

It is never wise to use the root account for non-system administration tasks. Because of this, for some usage scenarios it may be convenient for an unprivileged user to occasionally execute commands as the root user for system administration tasks without first either logging out as the current user or logging in as the root user. su and sudo are two tools that provide for this usage. A carefully designed executable with the suid-bit set might do the job more safely.

Root is also a generic term for the base of the Unix filesystem hierarchy, which is written as "/" in pathnames. There is also a directory named "/root" which is the home directory of the root user.