FWIW Nagios and Icinga are monitoring solutions. The plugins that were talked about so much are small pieces of software that check if enough free disk space o

FWIW Nagios and Icinga are monitoring solutions. The plugins that were talked about so much are small pieces of software that check if enough free disk space or RAM are available, that processes are running, ports are open etc. And then the core component aggregates the results from those checks, do alarming if configured, threshold checks (for example only five failed checks in a row result in alarming) and offer a web GUI where one can see all checks for a given host, get an overview of hosts with failing checks etc. I didn't have any contact with monitoring tools before joining an ISP, which relies heavily on them.