WHOIS query is widely used for querying databases that store the registered user's info for a domain name but is also used for a wider range of other information.
Is my information secure?
That depends on the type of domain name you registers. Most Top-Level Domain (TLD) name Registrars like co.za are privacy compliant and do not publicly expose the details of the registered owner of a domain, but not all are. Even if the registrar is compliant your details can still be requested in cases where for instance criminal or copyrighted infrindgments come into play.
Why should I keep my information private?
Some domain registration companies might provide an optional product that protects your details in a whois search, but again it depends on the type of domain you register. In most cases, you do not need to opt-in for additional security to protect your private info. Do some homework before you register a domain to determine if you need additional security for your domain.