Cluster is a group of servers that appears as single unit to the client. The advantages of a clusters are 1. Scalability : There are more servers to process requests. It is possible to add more servers without any client interruptions. 2.High avaliablity : This arises because of redundancy. If one of the servers in the cluster fails another one can take over.
Load balacing is a means of distributing client request among the servers in the cluster. We make of algorithms like DNS ROUND ROBIN, Hard ware load balancing for load balancing.
Clustering is about combining a group of servers into a single managed unit while load balancing is the act of delegating incoming load (e.g. HTTP requests) to one of the server instances in the cluster.