This whois interface allows you to retrieve information about registered domains in one of the 163 currently supported top-level domains. Just enter the domain name (for example trilithium.net or google.com) and press "look up". The script will automatically contact the relevant servers and display the information returned.
Note that the absence of whois information for a particular domain name does not necessarily mean that it is available for registration (it usually does, but there's no guarantee).