In Server 2008 R2 you can create a cluster of computers, which as I understand provides fail-over if a server dies. With Network load balancing I beleive this "offloads" some of the load to other servers, for example you could have two web servers with NLB...but couldn't you also have a cluster and NLB on the cluster as well?
Basically, where to use NLB and where to use clustering, for example, a database and a website.