I was looking for some information on what clustering is and how it can be used with J2EE. For people sharing the same interests, check out the links below
Abraham Kang writes elaborately about the basics of clustering, gives a good insight into different kinds of clustering and the clustering capabilties of different ApplicationServers. His list of advantages and disadvantages with different strategies is a highlight. Beware, the article is a little out of date ( writtten on 02/23/01), but the basics are solid.
Read his part II here to get the bigger picture on load balancing techniques and some tips (most of them already know these) on clustering.
An another introduction to clustering here.