Search engine optimization is one of those ongoing tasks. SEO only has two directions...forward or backward, and the day you stop paying attention to SEO is the day you start moving backwards.
If you are one of the so-called little guys, you may feel overwhelmed with how you can ever compete against the big guys. Well good news, as you'll see, even some of the big guys miss the mark on some of the most basic concepts.
Simply put, in regards to SEO, we might describe "canonicalization" as identifying and consolidating to one, definitive source. The most basic and simplest example of this is www.domain.com versus domain.com. In most cases, both of these lead to the same "page," that is, there is no discernible difference in content between the two. This doesn't have to be the case, but let's not worry about that for now.
This is so often overlooked because it generally doesn't present any noticeable issues. After all, your visitors and search engines get to your site with either version. It's important to understand that search engines see pages based on the URL, which means, to them, these are two different URLs, and therefore two different pages--even if the content is 100 percent identical.
On a basic level, this means duplicate content. Search engines have gotten much better about handling duplicate content and will eventually choose one page or the other to serve up. On a more critical level, what this means is that you may be dividing up your link popularity in all the engines and PageRank specifically in Google.
When it comes to link popularity and PageRank, you always want to consolidate your efforts. If you don't force all the PageRank you've earned through to one canonical version, you may split that ever important "link juice" between two different URLs. That's because some folks will link to your URL without the www, just out of convenience or laziness. This SEO issue is one of those simple basics that every site should take care of, especially if it ends up being the difference between one of your pages showing up above or below your biggest competitor.
So which do you choose? Whether you go with the "www" version or the "non-www" version isn't really an issue. What is important is that you use a 301 permanent redirect in order to redirect traffic to the version of your choosing. Then you'll consolidate and flow all of the link juice to the canonical version regardless of how others link to your site--rather than diluting or splitting it.
As you can see by the list below, there are still a lot of big-name sites that haven't addressed canonicalization. Each of these sites can be reached by their www and non-www URLs. Is this negatively impacting their rankings? Maybe, maybe not, but why pass up on the simple stuff.
Contrast the above with the following big-name sites that DO properly redirect: