Actually, excuse my earlier definition. What a "parked domain" is really depends on the context in which the term is used. Where did you hear it?
Sometimes it can mean a domain name that you buy but don't have a website for yet, so you "park" it with your domain registrar, which will make the domain point to a default "under construction" type page.
Jeremy » [email protected]
Front Drive » Advanced multi-domain hosting solutions
Or you can mask it. Meaning if you have good domain www.xxx and you have www.xtz parked, when people go to www.xtz, it will go to www.xxx and show the url of www.xtz.
Meaning people will see the url of the parked domain when it takes them to the url you are pointing at.
Much better then just a place holder page.