SolarWinds Server & Application Monitor (SAM) is primarily used for monitoring the performance, health and availability of commercial and opensource applications, in addition to custom application monitoring. SAM also monitors physical and virtual server health and performance. From things like CPU/Memory, disk volume utilization, hardware health, Disk I/O, network performance etc.
Network Performance Monitor (NPM) brings similar functionality to what SAM provides for Servers and Applications, to Network devices like routers, switches, firewalls, load balancers, and wireless access points. If you need to monitor your network infrastructure with the same level of scrutiny as your Servers and Applications, then NPM is an excellent choice. SAM & NPM are fully integrated solutions that complement each other, allowing for a single pane of glass view into your Servers, Applications, and Network infrastructure with centralized alerting and reporting.