问题
I've heard people mention the Canonical URL tag, but I don't know what it means or what its purpose is.
What is it and how does it affect SEO?
回答1:
Canonical URLs are used when you would otherwise have different URLs that point to the same content without actually redirecting. So let's say your domain is example.com; www.example.com and example.com are two different URLs, and if you don't redirect them to a standard (canonical) URL (for instance, example.com), search engines treat the two URLs as completely different pages. This causes the link popularity of the two URLs to be split, which is obviously very bad for SEO.
来源：https://stackoverflow.com/questions/1047438/what-are-canonical-urls-and-how-do-they-affect-your-seo