This answer will explain what the WHOIS database is, what information is shown on it, and how to view this information.
Why use WHOIS?
WHOIS is a public database which allows you to find out information about a specific domain name. You can find out who registered it, who the registrar is, and who you can contact about the domain name.
Searching the WHOIS is also a useful way to check whether a domain name has been registered. (This the method the 123-reg domain checker uses. If there is no result returned, then we attempt to register the domain name for our customers.)
How does the WHOIS database work?
The WHOIS database contains contact and registration information for every domain registered with a particular registrar. WHOIS databases are maintained by individual registrars and not by the central registry.
For this reason they are not always fully up-to date. Most of the systems are updated every 24 hours, which means the results that they return can be up to 47 hours out of date.
When you register a domain name with us, we pass on the contact details to the WHOIS. These will then be publicly available through WHOIS services. For most domain types this is a legal requirement. However, .uk domains administered by Nominet have an opt out system.
A useful site for checking WHOIS information is www.allwhois.com.