While the Linux operating system is very stable, operator errors, such as poor CGI scripts, bad configuration files, or faulty installation of additional software, can and will cause problems. And while rare, crashes and accidents do happen. Monitoring alerts you to problems with your server quickly , so that you can take immediate corrective action . It helps keep your business running , your customers and visitors happy and your profits rolling in. DNS Server verifies that the DNS server is working.
Database tests a database by connecting to it and performing a query.
FTP connects to the FTP server and verifies that a file can be retrieved. E-mail checks mail by sending a mail message, and retrieving that message. URL Transaction verifies that a transaction can be performed, by checking a sequence of web pages and forms. Ping determines whether the computer is running or needs a reboot. Port monitors whether a specified service is running as expected.