When you register a Domain Name, it is accompanied with a public record namely, WHOIS information for that particular Domain Name, which evidences the ownership of the Domain Name. On Registration of Domain Name, a Domain Registrant is subject to a Domain Registrant Agreement which mandatory requires a domain registrant / domain owner to maintain at all times, correct WHOIS information.
Therefore, it is quite important to maintain correct WHOIS information for a Domain Name. Once a Domain Name is registered the WHOIS can be checked at various website, most famous being http://whois.domaintools.com. Basically, WHOIS information contains three sets of data, one for Domain Registrant, another for Admin Contact and lastly the Tech Contact.
The Registrant reflects the Domain owner, while Admin and Tech details can be different, though in 90% of Domain WHOIS information you will find all the sets as displaying the same information. Though, for transferring of the Domain Name from one Registrar to another, the Transfer Approval email is send to the Admin contact for a Domain Name.