A network time server (commonly referred to as the NTP time server after the protocol used in synchronisation – Network Time Protocol) is a device that receives a single time signal and distributes it to all devices on a network.
Network time servers are preferred as a synchronisation tool rather than the much simpler internet time servers because they are far more secure. Using the internet as a basis for time information would mean using a source outside the firewall which could allow malicious users to take advantage.
Network time servers on the other hand work inside the firewall by receiving source of UTC time (Coordinated Universal Time) from either the GPS network or specialist radio transmissions broadcast from national physics laboratories.
Both of these signals are incredibly accurate and secure with both methods providing millisecond accuracy to UTC. However, there are downsides to both systems. The radio signals broadcast by nation time and frequency laboratories are susceptible to interference and locality, while the GPS signal, although available literally everywhere on the globe can occasional be lost too (often due to bad weather interfering with the line-of-sight GPS signals.
For computer networks where high levels of accuracy are imperative, dual systems are often incorporated. These network time servers receive the time signal from both the GPS network and the radio transmissions and select an average for even more accuracy. However, the real advantage of using a dual system is that if one signal fails, for what ever the reason, the network will not have to rely on the inaccurate system clocks as the other method of receiving UTC time should still be operational.