Don’t worry – it took me quite some time to say this properly too!!
This is probably the quickest but most effective thing you can do for your overall website health.
A definition of Canonicalization from Matt Cutts at Google “It is the process of picking the best url when there are several choices, and usually refers to home pages”.
What does this mean in English?
There are several ways that you can write a website address:
Yes I know what you’re thinking – they’re all the same and go to the same place. For a person yes – but not to a search engine. Technically all of these url’s are different and could show totally different content on each url which gets search engines very confused and cross (and ultimately gives you lower rankings).
So what to do? Pick one format that you’d like to use – either www or non www and update the header section html on your website with the following code:
(you can see here I’ve chosen to go with the non www version)
The version you choose should be the one you think is most important. If you don’t mind then choose that gets the most visitors or has the most backlinks. But choose one and be consistent!
There is very divided advice in the industry about www vs non www and you can read more about it here.