Monitoring a server means that the server owner always knows if one or all of his services go down. Server monitoring may be internal, i.e. web server software checks its status and notifies the owner if some services go down, and external, i.e. some web server monitoring companies check the services status with a certain frequency. Server monitoring can encompass a check of system metrics, such as CPU usage, memory usage, network performance and disk space. It can also include application monitoring, such as checking the processes of programs such as Apache, MySQL, Nginx, Postgres and others.
External monitoring is more reliable, as it keeps on working when the server completely goes down. Good server monitoring tools also have performance benchmarking, alerting capabilities and the ability to link certain thresholds with automated server jobs such as provisioning more memory or performing a backup.