When you have a few domain names with different extensions and you would like all of them to open the exact same website, you could have the site under one of them and forward the rest. There are different ways to forward one domain to another, among them the so-called domain parking. If your web hosting plan permits it, though, it shall be better if you host all the domains and set up a URL redirect, not a domain redirect. The main difference between the two is that while a domain address is hosted, you may still have content for it, create subdomains, email addresses, and so forth., while with a parked domain you can't do any of these things. For example, if you are building localized websites under various country-code domain names, you will be able to work on them, but in the meantime, people shall be redirected to the main Internet site.