You write a brief excerpt about an article on someone elses blogs. I always add links to the blog and story I'm talking about in the blog. Then, if using wordpress, below where you are typing you enter the trackback for the article you are commenting about. So if I wanted to comment on this article
http://www.xxxpornblog.com/blog/?p=82 I'd write about it on my blog then I'd add the trackback to my article on my blog. So I go to where it says "trackback" on the the article and copy the link location and add it on my article on my blog. Publish the article and people from my blog can read the article via the hardlinks I put in the story and people who are reading the article I was writing about will see a comment from me with a link back to my blog.