Apart from being a dreadful word, canonicalisation is an important factor for Webmasters and site owners to consider. Basically, it refers to picking the URL that you want for your primary URL and re-directing the ‘other versions’ to it.
Let’s say you have:
Although you would imagine they’re all the same, technically they are different. Potentially, a web server could show a different profile, e.g. links and content, for each of these scenarios.
Try to get into the habit of being consistent with your internal links.
More importantly, go to your web server and set-up a permanent (301) re-direct from the other versions of your domain to the URL format you have selected for your primary. Your visitors won’t notice any difference, but you can make sure that any link value that the alternative URLs have gained aren’t lost.
The search engines are getting better at resolving this issue and Google Webmaster Tools allows you to set a preferred domain. However, you shouldn’t rely on this and instead use a 301 re-direct to ensure that you control which view of your domain you’d like the world to see.