With a small number of hosts, the most useful screen is the Service Details which lists the status of everything that is monitored by Nagios.
The Status Grid provides a less detailed summary, and is suited to a moderate number of hosts.
The monitored hosts up/down/unknown status is summarized in the Host Detail page.
Service Problems provides a list of services that are warning or critical.
The Host Problems page lists all hosts that are down.
There is also a Network Outages page.