Wikipedia describes latency this way:

“Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. In communications, the lower limit of latency is determined by the medium being used for communications. In reliable two-way communication systems, latency limits the maximum rate that information can be transmitted, as there is often a limit on the amount of information that is “in-flight” at any one moment. In the field of human-machine interaction, perceptible latency has a strong effect on user satisfaction and usability.” (Wikipedia)

Given that it’s quite important for any developer to know his numbers. Since latency has a huge impact on how software should be architected it’s important to keep that in mind:

 

 

Source: http://www.eecs.berkeley.edu/~rcs/research/interactive_latency.html