A parked domain is an additional domain name which loads your main site. The main reason why people do this is when they are marketing specific keywords contained in the new domain name, require a shorter domain name or an easier one to remember.
Google considers parked domains as duplicate content and penalizes these sites in the rankings. The other search engines might not treat parked domain names in the same fashion as Google does. I personally would not use this practice because most of the hits on my site come from Google. The way to make this acceptable is to have the secondary domain names use 301 redirects instead. I notice that even Google suggests that you handle multiple domain names pointing to one site in this manner.