What exactly is the domain name "masking" or "cloaking" offered by some hosting companies, (as opposed to redirection), and what are the various reasons it should be avoided when setting up a new web site "on the cheap"?
It's bad for SEO because search engines don't like frames if that option is used. Also, the URL never changes in the user's address bar so it can be very confusing to users trying bookmark or reference inner pages in your site.
If you use two different domains for the same website, which is another way to do it, then you run into duplicate content issues and penalties with the search engines and, once again, potentially confuse your users.
When you are Domain masking you are hiding the actual URL/domain from the user. This is not good either for your users or search engines because you are putting on a mask not showing the 'real face'. Also as john said bots will treat your site like a duplicate content farm. Which is no good.
Cloaking on the other hand is a balck hat SEO tactic. It can get you in a lot of trouble if you get caught. Either way don't do it. think longterm. Cloaking is providing different content to your users and different content to search spiders. Which you can simply use the ip adresses of the search engines to manipulate. But both of them are a poison for a website. good luck
my 2 cents