Canonical is the word used to describe the one address that you want the world to go to when they look you up. The typical choices are whether to use www in front of your domain or not. The classic example follows:
Choosing what your canonical website address (URL) will be is totally up to you. It’s a preference and there’s no right answer. As you can see by looking up at your browser’s location bar now, eQualit.ie has chosen a URL without www. If you start taking notice of the other websites you visit, you’ll probably see that there’s no regular pattern. Google chooses www. The wordpress.org team chooses non-www. It really doesn’t matter. What does matter is making that choice early and sticking with it.
Considering the Apex
One unique factor (with respect to hosting on) in your decision-making process is whether or not your domain will be hosted by a DNS company that supports pointing the non-www (officially called the apex record) address to a CNAME. If your DNS host does not support this feature, we recommend you choose www to be your canonical website address.
Here’s an article at Google Webmaster Tools called Use Canonical URLs that will help you to learn more about their view of canonical URLs.
Also, Matt Cutts provides some very helpful insight and a FAQ about SEO and URL canonicalization.