NLBS is intended for applications with relatively small data sets that rarely change (one example would be web pages), and do not have long-running in-memory states.
One attractive feature of NLBS is that all servers in a cluster monitor each other with a heartbeat signal, so there is no single point of failure.
For example, if a web server is returning an error page instead of correct content, it is still perceived as "alive" by NLBS.
As such, a monitoring script is typically required on every participating node, which checks the correctness of local web page delivery, and calls the nlb.exe utility to add or remove itself from the cluster as needed.
WLBS was replaced by Network Load Balancing Services in Windows 2000.