Want to know about canonical url

What is canonical url? What is duplicate content?Do you know that?I think most of us aware of this words and they too aware of this important for SEO of you website.
What I know is canonical url is using by search engine,when your website has more than different link for same content.Though most of them links created behind your awareness.As you don't know about them,when and how they come and affect your content with duplicative level.If you have a post about anything,then for this you have created a link for that particular post.Now think about your "comments" link at the very post.It will too redirect to your same post.But both links are different.But search engine will think that you have same content in two different link.Its called duplicate content.here you need canonical url to say search engine that except main link rest are nothing.For more details read my post from here.Duplicate Content.