Services are one of the prime reasons to install State, they are the whole reason for networks of computers. If machines are unable to access service such as the web or mail, then people might as well go on vacation or something. Each service can be in one of three states, NORMAL, WARNING, or CRITICAL. Of course, you'd rather they all be NORMAL. It is up to you which services are checked and the properties to use when checking them. Checking a service yields a bit of information on them depending on its status:
Things are just fine with this service! Typically the time the service took to respond is provided, along with any other identifying information the service provided during the check, such as version numbers. We all wouldn't mind having our services like this all the time.
The service could definately be better off. Typically this means things are just unhealthy, and that the service is just nearly FATAL. It's useful for non-serious things that could quickly become serious. Each Vital that is in this status will have a descriptive message giving some ideas concerning why the service is not up to par, but isn't that a good thing?
Drop everything and check on this service! This service is pretty much unavailable and people will begin complaining soon. Just as with the WARNING status you should check the Vital's note for information on the problem.
If you see service check yield statuses other than these, then something may be a miss and you should definately check on things, or at least the service checks documentaion. State includes a large number of built in service checks that can be used to ensure things are running smoothly and to keep you from having to write them yourself.
Many of the service checks can be executed using SSL, which allows you to check the secure forms of the services, such as HTTPS or SIMAP. You may also specify the port number the service is on, just in case they are non standard. In other situations, like NIS/NFS, all you do is specify the node and off the test goes.