SNTP or simple network time protocol is a simplified version of NTP (Network Time Protocol). Many of the complex algorithms utilised by NTP servers to maintain a precise time are not present in SNTP.
NTP is too complicated for implementation on many systems. Therefore SNTP is utilised when such complexities are difficult to implement or are not required, such as in embedded systems.
SNTP is most often used at the 'leaves' of a network where accuracy and precision are less of a concern.
NTP and SNTP are entirely compatible in terms of the packets of information transmitted between server and client. Therefore an SNTP based client can quite happily synchronise to an NTP time server.

Additional References:
NTP servers
NTP time server
SNTP server
Network time server |