A lot of persons have been asking this same question, well i will be as precise as i can be in explaining this.
Domain parking is a process where you register a domain and do not enter the name servers of your hosting company. Usually the domain registrar will park your domain by default on their server, this means that if you type your domain name into a web browser you will arrive at a default page provided by the registrar. If you are going to hold onto the domain for a while or not develop a site straight away then you may want to consider a domain parking service that offers you revenue from your parked domain.
Parking services such as sedo offer revenue sharing from your parked domain. Revenue sharing from a parked domain works by traffic being directed to a search page or pay per click page where any visitors to your parked domain may click on the ads and you receive a percentage of the revenue from this. Traffic parking claims that when parked receives on average 8 hits per month but a popular domain name may get thousands of visitors due to direct type ins and miss typed domains. However to get traffic to your packed domain name, us this traffic system that send 1000+ real visitors to your site.
Unless you have a popular domain name it probably isn't worth the effort to use a revenue sharing parking service and just use your registrars default parking page. So domain parking is a process where you register a name and leave it until you are ready to use it. You may or may not have a parking page where any stray visitors will see a default host parking page
If you intends to us google adsense for domain, simply register your domain name, then log into your google adsense account and select the google adsense for domain and then add you domain name and google will have it parked for you with some adds that will generate you revenue.
i hope this has been usefull, let us know if you have any problem with this.