A "parked domain" usually refers to one that is waiting for a website to be built. Often the registrar will show a single webpage for that domain that says something like: "JohnsWebsite.com is coming soon." This is often free and certainly isn't worth paying extra for, though some domain registrars may try to charge you for it. Sometimes the term "parked domain" is used to refer to a secondary domain name that "points" to your main website, so if someone types in the "parked domain" URL they'll end up at your main website. Your web hosting service might charge you $10 to $25 extra per year for an extra domain name pointed to your website.
An addon domain is a domain that points to a folder within your web folder. It's not so full-featured , but it does allow you to host more than one domain on a hosting account.
Sources:
http://answers.yahoo.com/question/index;_y...d=1006041309898http://answers.yahoo.com/question/index;_y...20024620AAj7AN5