Another definition for new people -

Domain parking is an advertising practice used primarily by domain name registrars and internet advertising publishers to monetize type-in traffic visiting an inactive domain name. The undeveloped domain name will usually point (redirect) to a page full of advertising links. These links will be targeted to the predicted interests of the visitor. Usually the domain owner is paid based on how many links have been visited (e.g. pay per click) or how beneficial they have been. The keywords of the domain name provide clues as to what the visitor sought before arriving. For example, the domain fast-weight-loss.com might point to a page of links to fad diets and drugs.

Another use of domain parking is to be a placeholder of an existing web site. A company might choose to use this method to redirect its web site traffic to another web site it owns.

SEDO is a good parking program to use.
I know people who make ALOT of money of this.
Of course youd probably make more if you made a site on the domain!

Typically parked domains just have google adword ads on their site.

You cant have adsense on a site with no content, it just wont work. Most people have sedo or similar on a parked domain.
If its got adsense and content, then its not considered parked

Yes Homer is right.
Domain parking is described well in this thread, familiarise yourself and reap the benefits