A time server is a server computer that reads the actual time from a reference clock and distributes this information to its clients using a computer network.
A variety of protocols are in common use for sending time signals over radio links and serial connections.
The NTP homepage provides a free and widely used reference implementation of the NTP server and client for many popular operating systems.
The term "stratum" is used to label the closeness to a central or high quality time server.
The scale is 1 to 15 where 1 is the most accurate and likely a highly specialized physical hardware device.