Renu, if you're completely serious at all, read up on it yourself. Stop making these threads because it seems pretty obvious you really don't know what you're doing.
I'm not trying to be mean, I just don't think you should make all these threads.
Fine, you want an answer?
Clustering is grouping computers together so that they work as one computer. The most famous kind is a Beowulf cluster, which generally has a bunch of machines running a Linux system with the Beowulf software.
Anything else I'm not quite sure about. MS has something called MSCS for clustering.