Nagios provides multiple services and I've listed few of them down:
Monitoring of network services such as SMTP, POP2, HTTP, NNTP, ICMP, SNMP, FTP, SSH
Monitoring host resources such as processor load, disk space, disk usage, system logs on many Operating Systems like Windows and Linux.
Monitoring hardware properties like temperature, alarms,
Nagios provides a mechanism to monitor remote systems using SSH.
Parallel service checks and automate log file rotation
A web-interface for viewing status, notifications, alerts, history etc.