A watchdog is a device used to protect a system from specific software or hardware failures that may cause the system to stop responding. The application is first registered with the watchdog device. Once the watchdog is running on your system the application must periodically send information to the watchdog device. If the device doesn't receive this signal within the set period of time it would execute the proper keystrokes to reboot the machine or restart the application.
Watchdog services are also available for Internet Web sites. In these instances the watchdog may be set to monitor your own Web site by attempting to access it from several different city or country locations at regular intervals. You could then view an online report of the connectivity or have the watchdog system e-mail you should it become inaccessible.
Some Web sites also offer readers a watchdog service where they receive instant e-mail notification when the Web site offering the service has been updated.