Clustering is a term that means that there are a set of computers (web servers) that will operate together such that if one fails (melts down, explodes, hard drive crashes, etc.) the others will continue to operate to service web requests and the web site will continue to operate normally.
Also, when web servers are clustered they typically have some form of load balancing in place. A load balancer sits between the web servers and the Internet. Its job (in theory) is to make sure that no web server becomes burdened with too many requests. If one server is starting to become overloaded, it will move subsequent requests on to the other web servers in the cluster.
If a company gives you a set of domain names (blah.com, reallyblah.com, blahblah.com etc. etc.) it means that users can visit any web site that those domains are mapped to.