Clustering just means having more than one server (i.e. horizontal scalability rather than vertical). DNS Round Robin is a load *sharing* technique rather than load balancing. Requests are shared out to a list of servers, one at a time. Pretty dumb ... even if one server fails, requests will probably continue to be sent to it. Reverse Proxy load balancing means sending requests to servers according to the type of the request. So for example, all static HTML requests go to server X, while all requests for this compute intensive service go to server Y because it's bigger. HTH.