A cluster is a group of database servers your LiveJournal is stored on. I am on the 'Santa Cluster'. You can find out which cluster you are on by visiting the "Where are you?" page.
LiveJournal also names some of it's servers after South Park characters. Although most servers are named with colors or 'ws#', you may notice in the headers of your comment notification e-mails that they came from the Pip, Sheila, Hankey, Wendy, etc. servers. You normally can find this by clicking on 'view all headers' in your e-mail client. update- you do not have a permanent email server. You can recieve comments from any of the server names (unlike the cluster which does not change).
It's pretty nifty to get an e-mail from Mr. Hankey the Christmas Poo, wouldn't you say?