NRPE and NSCA in Nagios

NRPE is used for performing active checks in Nagios
NSCA is used for performing passive checks in Nagios

check_nrpe (from NRPE) is used to initiate an active check from the  Nagios server. check_nrpe is called like a normal plugin by nagios server. check_nrpe initiated in the server, then contacts the nrpe daemon running on a  remote host(a host monitored by Nagios) and asks it to run a pre-configured check command and  returns the results to nagios server.

NSCA is used when a passive check is initiated from a remote host(a host monitored by Nagios server). In the remote host, a  wrapper script for checking a service, is called by cron or some other method,  which in turn  passes the results to the send_nsca  program running in the same remote host. send_nsca contacts the nsca daemon on the nagios server. nsca daemon passes the results of the check to nagios.