When you have more than one domain and only the one web site that you want them all to point to you have three choices as to how you set things up. The way these three alternatives work relates to which of three different HTTP codes that gets returned in response to the request to access the web site using any specific domain.
200 is the value returned by HTTP when any web request runs successfully. This indicates that the web page requested by the specified address has been returned successfully.
To use this approach to handling the multiple domains it is simply a matter of setting up the web site on one of the domains and then parking the other domains on that same site. All parking a domain means is that you are pointing it to the same location as another domain. When you do this the pages will be accessible via any of the domains and will display as part of the domain specified. As your visitor will not know that the domains point to the same web pages the pages will look like someone has copied them from one domain to the other. The pages are effectively duplicates. As far as the search engines are concerned all but one of your domains are displaying duplicate content and should be ignored. Unfortunately the links to the site using those addresses will not count as adding authority to the site because the search engines will see them as separate sites.
To overcome the problems of this first approach we can use either of the other two. Both of these start with our setting everything up as we did for this first approach but now we add a redirect command that redirects all the requests to access one domain to access a different domain instead.
302 is the value returned by HTTP when a web request runs using a temporary redirect. This means that all requests to one site are redirected to the other. That second domain is the one that then appears as the address of the page. This address though is defined as being only temporary. It indicates an intention to eventually return to the original address. The web pages are then considered to belong to the first domain with the second only representing a temporary replacement. The problem with this approach is that all your real visitors see the site on one address but it isn't the address that has been identified as the site's permanent address. This has additional problems regarding those who access that temporary address directly as to those visitors this is the permanent address of the site while to everyone else it is only a temporary address.
The better alternative is to use a 301 permanent redirect. Now it is the destination of the redirect that is the permanent site and the others are there either to provide a convenient shortcut to access the site or are perhaps prior addresses for the site which are being phased out. This means that everyone sees the same address and it is the one that is considered to be the permanent location for the site. The other domain names become merely alternate ways for people to access the site (useful if they are common mis-spellings or alternative descriptions that people are likely to try), a short but relatively meaningless alternative that can be used when you need to specify a short URL, or (and this is perhaps the most useful reason for getting the other domains) because it prevents others from setting up competing sites using those domains.