NTP (Network Time Protocol) is the most prevalent time synchronisation software available. On of the reasons NTP is so successful is the way it organises its clients into a hierarchy.
The hierarchy of NTP is divided into stratum with each strata representing the distance from the original reference clock. For instance an atomic clock that generates a UTC (coordinated universal time) signal is referred to as a stratum 0 device.
A NTP server that receives a stratum 1 time signal is referred to as a stratum 1 device and a device that receives a time source from a NTP server is a stratum 2 device. NTP can support up to 16 strata although the further away from the reference clock you get (stratum 0) the less accurate the device will be.
However, by arranging the network into stratum and allowing stratum 2 devices to pass on the time to a stratum 3 device (and so on) it reduced the demand on the NTP server and the network. By using a stratum based network, realistically thousands of machines can be synchronised to just one NTP server.