Search engines are picky creatures. They do not always understand your intent, and will often penalize your site if it does not follow its rules, even if your intent was good. Using multiple domain names to point to one site is a great example. There are so many ways (many of them bad in the eyes of the search engines) to handle this problem that the average webmaster is often doing it wrong without ever realizing it. And most of the time, the webmaster is simply doing what appears to be the right way - the way that the hosting company says to do it - or the way that the domain registrar says to do it. But, alas, the hosting companies and domain registrars are not necessarily leading you down the correct path (or they are not leading you down the search engine approved path, I should say).
Incorrect handling of multiple domain names can lead to duplicatation penalties being applied to your site. If you have chosen the incorrect path, but you have not yet been penalized for it by the search engines, then you may not want to make any changes yet. But keep this information handy somewhere, just in case you ever need it. If you wake up one day and wonder where all your lovely rankings have gone, you can use this information in case you decide the problem lies in your mishandling of domain name redirection.
Rather than attempt this explanation myself, I will point you to the clearest example I could find on the internet to explain How To Redirect Multiple Parked Domains. Jill Whalen does an excellent job of keeping the explanation simple and easy to understand for everyone, which, for this subject, is nearly impossible to find on the net. Read it - and then either use it or bookmark it for future use.