NCPA is a cross-platform monitoring agent for Nagios Core and Nagios XI that runs on Windows, Linux/Unix, and Mac OS/X machines. Its features include both active and passive checks, remote management, and a local monitoring interface.
NRPE (Nagios Remote Plugin Executor) is an addon allows you to remotely execute Nagios plugins on remote Linux/Unix machines. This allows you to monitor remote machine metrics (disk usage, CPU load, etc.). NRPE can also communicate with some Windows agent addons like NSClient++, so you can execute scripts and check metrics on remote Windows machines as well.
For more info refer https://assets.nagios.com/downloads/ncpa/docs/NCPA-Agent-Comparison.pdf