Microsoft Knowledge Base Article 303751 contains the following summary:
This step-by-step guide describes how to install Network Load Balancing (NLB) if it has been uninstalled.
You can use NLB to spread incoming requests across as many as 32 servers. This lets you expand enterprise-wide services, such as Web servers, Terminal Services, and streaming media servers, and still ensure that the servers will respond quickly, even under heavy loads.
NLB automatically detects if a server stops working and quickly reassigns client traffic among the remaining servers. This safeguard can help you provide uninterrupted service for your critical business programs.