One Website, Multiple Domain Names (URLs)
We're working with a situation now where our client has multiple domain names (urls) that point to the same website. In other words, there is one directory folder that holds all the website's files, and several domain names point to this same resource.
In some ways this is a relief because the alternative could be that there are several different website folders that each domain points to. The fact that the different domains all share one file source at least means that when the files are updated the updates appear to all visitors no matter which url they arrived through.
What are the SEO Implications?
The downside is that from a search engine optimization point of view, the urls are being treated as different sites. So as we build a backlink strategy we must be careful to only use one url or the danger is that we'll lose the impact of many inbound links if the links are spread out over the various urls.
How to Align Multiple Domains (URLs) Sharing One Website for Maximum SEO Impact
Google Webmaster Central on URL
Leave it to Google to be aware of this issue and offer some relief, http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html.
Wikipedia on canonicalization
And for more technical information about normalizing or canonicalization see this Wikipedia entry, http://en.wikipedia.org/wiki/URL_normalization.