A cluser, ideally, is a group of like machines (same h/w config) running the same application for scalability and failover purposes. Same application copied/cloned/installed across a set of redundant servers with the same resources available on each server.
A load balancer is used to "spray" request requests across the cluster members (or send requests to any number of different servers). The load balancer may also be responsible for directing requests to a particular server if the the request has an affinity to a particular server. The load balancer may also be resonsible for identifying servers that are not responding for some reason and redirect a request to another server in a cluster (for failover purposes).