The same (or pretty much the same) content is often found at different URLs. Ideally, everyone would use one and only one URL to point to that content. That would be a canonical URL. It would make it easier to retrieve backlinks. Ideally. Yeah.

Google, Microsoft and Yahoo let webmasters specify canonical URLs for their content using <link rel="canonical"> - as long as the URL is in the same domain.

I think Digital Object Identifiers are meant to provide canonical URLs, but I'm not sure. Many scientific articles have (only one?) DOI.