Web sites are found by their addresses on the World Wide Web. These addresses are known as URLs, or Uniform Resource Locators. Every website has a URL assigned to it, so both searchers and Web servers can find them quickly and easily.
Cluster analysis or clustering is the task of grouping a set of objects in such a way that objects in the same group (called a cluster) are more similar (in some sense or another) to each other than to those in other groups (clusters).
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, in fact, contain valuable data.