A cluster is a group of disk sectors. The operating system assigns a unique number to each cluster and then keeps track of files according to which clusters they use. Occasionally, the operating system marks a cluster as being used even though it is not assigned to any file. This is called a lost cluster. You can free up disk space by reassigning lost clusters, but you should first make sure that the clusters do not, contain valuable data.
The cloud has become the new computing cluster. The ability to have two or more computers operate in parallel for highly available systems has been around and is continues growing in the recent days.
Clouds and clustering are becoming more interrelated and complementary for IT managers. While the two technologies have come from different heritages and infrastructures, they are now merging to provide a powerful tool for managing complex enterprise workloads.