xntpd is the Network Time Protocol version three daemon software. The "x" was added to the name because the branch of code that eventually became NTPv3 was "experimental". The name of the software was changed back to "ntpd" for version four because the NTP creator, Dave Mills, decided that something probably shouldn't be "experimental" for about twenty years without changing dramatically.
NTPv3 is an internet draft standard. See NTP for why it's not a full standard.
NTP is an excellent software package for maintaining very accurate time on a personal computer. It is easy to set up, and incredibly accurate. Distributions are available for every major operating system, and most minor operating systems.
xntpd (Version 3) supports private key cryptography for keeping things secure between the client and server. ntpd (Version 4) includes public key cryptography that is considerably easier to set up and maintain. For more information on NTP, see the NTP node and the web site at http://www.ntp.org/