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 does not 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 may not be aware of it, because the other servers in the cluster can take over the sessions it was handling.