The uber-most super user. UID 0 is the root user of any Unix system. The head honcho, the ghost with the most, the source of all good bits, Jason.
Each user must also have a unique UID (user ID) number. This is an integer between 0 and 32767, although some systems permit numbers up to 65534. In networks using NFS the UID must be unique across the network, as well as being attached to the same username and person. UID numbers are conventionally assigned to human users beginning with 100. Numbers less than 100 are reserved for system accounts. Some UID numbers are assigned to system accounts during the installation of the operating system. What numbers are assigned to what accounts will vary between types of Unix. Some typical system accounts and UID's are listed below.