OK, this one's going to sound a bit geeky, but it's important enough that you should try to follow it through. The major search engines have announced that they are recognising a new tag which will help many websites, and which will probably result in some big changes in the search engine rankings. However, I suspect few will get around to implementing this new feature, so there's a competitive advantage if you do.
Here's the problem. You might think that each page on your website only has one address, or "URL". But that's probably not the case. For example, if your website is set up correctly, you should be able to drop the "www" part of the address, and the "index.xxx" too. So on our website, all of these addresses will take you to the same page:
There are many more possibilities on some websites. A few of you may be using subdomains, where something is substituted for the "www" part of the address; for example, software supplier Adept Scientific set up endnote.adeptscience.co.uk to point to the product page for its EndNote product, because it looks nice, but the page is really on the normal "www" subdomain at www.adeptscience.co.uk/products/refman/endnote/. Or perhaps it's the other way around?
Still with me? Good. It'll be worth it.
Now, why does all this matter? Well, what happens if Google thinks that that all of those addresses above are separate pages, when they're actually the same one? As you know, one way Google assesses a page's importance is by the external links to it. So you might have ten other sites linking to a page, but those links might be to various different ways of expressing the same page. If Google thinks those pages are different (when they're not), the value of the links is diluted. Wouldn't it be good if you could just say "look, this is the specific URL of that page I want you to link to" and to get all those external links to point to exactly the same URL? It would be good, but it's not going to happen.
And there's another problem with these multiple URLs for the same page. In extreme cases, Google might think you have the same page on your site loads of times. This makes the site look like the sort of thing spammers create (even though it isn't), and you can be "penalised" for it in the Google results.
So what the new "Canonical" tag does is to allow you to say to the search engines: "This is the URL I want for this page. Please treat any other URLs you have which end up here as the one I'm specifying". That way, the value of all the external links is consolidated onto one URL. Depending on how your site is set up, and if it has been suffering from external links pointing to different URLs for the same pages (probably without your knowledge), you might see an improvement in your search engine results positions.
The tag is hidden away in the code for your page (like the "description" tag and others), and there's more information on how to do this on the Google Webmaster Central Blog. It's important that you forward this information to whoever manages the IT side of your website.
To see an an example, this page has one of these tags in, if you view the page source (it's about 20 lines down, and indented). Thanks to yoast.com for the WordPress plug-in which has enabled me to add the tag to hundreds of pages in seconds.