A cluster is a set of WebSphere servers working together, serving pages at the same port. From the user’s point of view, all of the servers function as a single server; it doesn’t matter which server handles a given request.
Virtually all production sites use clustering. Clustering provides much better performance and reliability than running on a single server. For example, if one server in a cluster goes down, the user will not be aware of it, because the other servers in the cluster can take over the sessions it was handling.
