All registered domains are listed within the public WHOIS database. The WHOIS database may be viewed as a giant phonebook of all domains. When someone is looking for information about a particular domain, their first point of reference is usually a WHOIS lookup.
As all information within the WHOIS database is publicly available, care should be taken over what data is supplied. Certainly, if an e-mail address is listed in a WHOIS entry, there is a reasonable change that it may be harvested by spammers. Likewise for personal address details.
Protecting your details
For many domains, it is possible to protect individual registrants' details (the domain owner) by enabling domain privacy.