What Are Canonical URLs?

So I was setting up a new wordpress blog and configuring the All In One SEO wordpress plugin when I came across this canonical url thingy which I haven’t been paying too much attention about. So what is that exactly? The short summary is that it’s a html tag you can enter in the section that helps the search engines to determine which url they should be indexing if they can reach your site through different but similar url. Google Webmaster Central has a longer explanation which you can read here.