People can use a "whois" service to determine who is registered as the owner of a domain. However, domain registrars often provide services to protect the privacy of the domain owner, and registrars have various processes for requesting this information.
Domain names are owned by individuals or companies that register the domain for a certain period of time. This information was originally supposed to be available for anyone to find, so the whois tool, which is now accessible through various websites, became the de facto means of finding this information. In addition to stating who owns the domain, this tool also states when the domain was registered, how long it's registered for and when it's due for renewal.
Due to concerns about privacy, registrars now often provide anonymity services. Instead of listing the name of the user who registers the domain, these registrars provide contact information for the registrar itself. Some registrars provide contact information upon request, but others demand documentation before releasing names. In some cases, the requester might need to provide a subpoena or other legal document demanding that the registrar release the name of the person who owns the domain. Legal requests that cross national borders can complicate this issue, and some registrars might simply refuse to release the requested information.