...For services a connection will be made to the entered ip or domain, on the given port. This way you can check if certain services on your machine are still running. To check your IMAP service for example, enter port 143.
For websites you can enter a link and it will check the HTTP status code. If the HTTP status code is in the 4xx range or a timeout, it means an error occurred and the tool will start sending out notifications. You can also set a regular expression to match for content on the page itself.
Each server has its own settings regarding notification. ...