Network Latency is a lag or a delay in the
network. It can be used to describe the amount of time it take a
data packet to get from point A to point B.
There are several things that cause network latency :
Transmission method and speed, Propagation, Router processing , storage
time on various machines through out the network, etc etc.
You can measure it by sending data from to controlled servers and seeing how much time it takes to get there.
Hope this helps.