There are many who don't realize that when you register a domain name, you have to divulge certain personal information about yourself. ICANN, which regulates domain names, requires that the owner of a domain provide their name and contact information for Whois lookup. By default, that information is publicly searchable. Although it's possible to shield yourself by using a domain name registrar that supports Whois privacy, many people can’t be bothered to protect their privacy, which means curious minds can learn a lot about the person or group behind any given domain name. If you're interested in learning more about a site's owner -- whether you want to buy it from them, or just learn more about them -- read on to learn the essentials of Whois.
One field of the Whois database shows the status of the domain you're interested in. If you want to buy a domain and transfer it to your own registrar, this field will show you whether it's possible. A domain listed as Transfer Prohibited can't be reassigned to a new registrar. Policies vary; some domains are only restricted for a given amount of time, and then can be moved freely.
Even if an owner has masked their identity, the nameserver of the domain can reveal some useful information. Most often, the nameserver belongs to the company that hosts the site. Contacting that company can help you find out whether the site is for sale.
If there's a domain you'd really like to own, like yourname.com, but someone else is sitting on it, there's always a chance you can get it for yourself without buying it from the current owner. Many people speculatively buy domain names, hoping they will become valuable in the future. If nobody bids on them, the speculators may let their registration lapse rather than renewing it. If your desired domain name's registration is expiring soon, and it's not of high value, try waiting to see if the registration lapses and you can snap up the domain name on the cheap.
If you already own domains, you're at risk of being spammed or scammed by people who skim the Whois database for real identities. Use a Whois lookup tool to see whether they have Whois privacy or not. If they do, the registration information will show a forwarding service rather than your actual name and contact information. If you do see your personal information, consider purchasing or enabling Whois privacy on that domain to protect yourself.