Yup, you read that right. Canonicalization. Seriously. It's a word. It refers to the art/science of making sure that search engines are associating the correct URLs with the pages on your web site. You see, search engines like Google aren't very good at telling when similar URLs actually refer to the same page.
For example, Google could potentially treat "www.abc.com", "abc.com", and "abc.com/" as three completely different pages even though all those URLs obviously go to the same page. Think about how much of a problem this could be for SEO. Instead of having one well-respected page, search engines think that you have three totally different pages, and none of them will be as likely to show up in search results. That's where canonicalization comes in. By properly canonicalizing your site, you can make sure that search engines don't split up your SEO juice across multiple identical pages.
There are a lot of different ways to approach canonicalization, but here are three easy things you can start doing:
Use 301 Redirects
Sometimes you'll be forced to have different URLs that resolve to the same page. For example, let's say your site has an affiliate program and you want to credit your affiliates with referrals. They might be able to include a parameter in the URL of all their links so that you can link the visitor to the right affiliate, but that URL parameter makes Google think that each affiliate is linking to a different page (so you're SEO juice is diluted). One way around this is to use a "301 redirect" to forward the user to a page without the variable in the URL. 301 redirects tell search engines that a page has permanently moved so SEO juice is passed on to the new URL.
For example, someone might link to your site using the url "www.FakeWebsite.com?AffiliateId=47". When a visitor clicks that link, you can store the Affiliate Id in the visitor's session, and then forward them to "www.FakeWebsite.com". This makes Google realize that the Affiliate Id shouldn't really be counted as part of the URL.
Forward all your pages to www
I've mentioned this one before, but I think it bears repeating. if you allow people to visit "www.yoursite.com" and "yoursite.com" and they both work the same way, you're running the risk of Google counting them as two completely different websites with identical content. Avoid this problem by automatically appending the "www" to all pages on your website. You can learn how to do this in the "Redirect to www" section of the page I linked to above.
Use the "Canonical" HTML link tag
Last year, the major search engines agreed to let webmasters indicated what the real URL should be for any web page. Going back to the affiliate example, you could leave the parameter in the URL, and instead tell search engines what the master URL is for that page (without the URL parameter). You do this by adding a "canonical link tag" in the head of your html file which points search engines to the master URL.
You can read Google's explanation of how to use the canonical link tag in this blog post. It's worth noting that specifying your canonical is really just a suggestion to search engines. You shouldn't try to use this to trick Google into combining the SEO juice of multiple pages into one page, because they will most likely start ignoring your canonical suggestions at that point.
So that's all for now. I plan on talking about canonicalization more in the future, but hopefully these three tips are enough to get you started.