Network Load Balancing is a part of the recent Windows server generations. It is used to balance the TCP/IP traffic among multiple members (e.g. web servers, streaming servers, VPN servers or terminal servers) in an NLB cluster.
There can be up to 32 members in an NLB cluster. NLB distributes the incoming client requests and directs them to one of the members of the cluster. If any of the members goes down, NLB stops forwarding requests to it.