As you are all most likely aware already all domain registrars ask for your personal information.
Such personal information includes:
- Forename and surname
- Address information
- Email address
- Telephone number
Personal information via WHOIS
Depending on the domain type such as TLD or ccTLD this information may become immediately available after the registration of the domain via a simple whois. If you do not want that (and in most cases you certainly don't) they offer a service where you can remove this information from the whois service but of course hiding your personal information adds yearly costs to the domain purchase.
What happens if I use fake details when registering a domain?