What is difference between SNAT and NAT in F5?

See this link for an explanation.

SNAT is Source NAT. Usually, initiated from one direction depending on configuration.

NAT - Network Address Translation

NAT is usually utilized to provide access to non-routeable addresses (RFC1918). Traffic flow initiation is bi-directional.

Internet Client <> Public IP <> {NAT Translation} <> Private IP

There are difference types of SNAT as noted here with the reasons for using them.

SNAT is usually utilized when you need to translate the original client IP to that of the SNAT IP. It is also used to make sure that the return traffic is passed to the F5 instead of following an asymmetric route/pathway away from the F5.