What is the difference between Application Warning/Critical State & Application Down?
Application Warning and Critical states manifest when a measured value passes a limit (threshold) that you've set. For example, if you have an Apache application monitoring an Apache server, you might set thresholds to notify you when your web server starts getting hammered.
You might set it up so any ServerRequestPerSecond value above 100 should merit a Warning, and any value about 150 should merit Critical.
Warning threshold: ServerRequestPerSecond >100 but less than <150
Critical threshold: ServerRequestPerSecond >150
What would an example of Application Down be?
Your Apache web server has crashed.