What is a canonical tag?
A canonical tag (aka ?rel canonical?) is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or ?duplicate? content appearing on multiple URLs. Practically speaking, the canonical tag tells search engines which version of a URL you want to appear in search results.
Why does canonicalization matter?
Duplicate content is a complicated subject, but when search engines crawl many URLs with identical (or very similar) content, it can cause a number of SEO problems. First, if search crawlers have to wade through too much duplicate content, they may miss some of your unique content. Second, large-scale duplication may dilute your ranking ability. Finally, even if your content does rank, search engines may pick the wrong URL as the ?original.? Using canonicalization helps you control your duplicate content.