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The Network Time Protocol (NTP) defines a set of procedures for synchronizing clocks on hosts connected to a network with access to the Internet.
Some of the hosts act as time servers, that is, they provide what they believe is the correct time to other hosts. Other hosts act as clients, that is, they find out what time it is by querying a time server. Some hosts act as both clients and time servers, because these hosts are links in a chain over which the correct time is forwarded from one host to the next. As part of this chain, a host acts first as a client to get the correct time from another host that is a time server. It then turns around and functions as a time server when other hosts, acting as clients, send requests to it for the correct time.
The network time daemon ntpd(ADMN) is the program running on each of the hosts in the network that, following the conventions defined in the protocol, attempts to establish the correct time. It does this by using the best available source of time to synchronize the host clock with the time at zero longitude, known as Coordinated Universal Time or UTC.