Microsoft Knowledge Base Article 303608 contains the following summary:
This step-by-step guide describes how to configure Network Load Balancing (NLB) parameters.
Network Load Balancing lets you 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 they respond quickly, even under heavy loads. NLB automatically detects when a server stops working and quickly reassigns client traffic among the remaining servers. This safeguard can help you provide uninterrupted, continuous service for your critical business applications.