Triggers are things that fire. They look at item data and raise a flag when the data does not fit whatever condition has been defined. As mentioned before, simply gathering data is nice, but awfully inadequate. If you want any historical data gathering, including notifications, there would have to be a person looking at all of the data all of the time, so we have to define thresholds at which we want the condition to be considered worth looking into. Triggers provide a way to define what those conditions are.
Earlier, we created a single trigger that was checking the system load on A test host. It checks whether the returned value is larger than a defined threshold. Now, let's check for some other possible problems with a server, for example, when a service is down. The SMTP service going down can be significant, so we will try to look for such a thing happening...