If you choose not to protect your domain with ID Protect, the name, address, and phone number you specified when registering your domain will be publicly available.  ID protect hides your registration information which protects your identity from spammers, stalkers and fraudsters.  ID Protect also helps protect you from being solicited by those using the Whois data illegally, much like an unlisted phone number. 

Please note that not every Registry will allow ID Protect on domains.  For example, .US domains are prohibited from using ID Protect.

Please also understand that if ID Protect is not ordered when a domain is first registered, and added later, then during the interim where the Whois information is not protected there are third-party sites that will retain the real Whois information even after ID Protect is purchased.  It is up to the owner of the domain to contact these third-party sites and try to get the sensitive information removed.