Trackback is a protocol supported by some blogging systems (see also the section on Blogs) for establishing a "reverse link" to e.g. an item on a blog. Unlike normal links, such as this one, trackback is initiated not by the author of a page, but by the author of another page requesting a link back to it. It is currently mainly used as a "remote commenting" system. Instead of responding to an item on a blog using the blog's comment feature, another blogger might write a comment on their own blog, and then send a trackback to the original blog (so that readers can click on a link to see the comment). The advantage of remote commenting is that the author of the comment has control of what they've written and are able to see it as part of their collected works, rather than as something they may never see again.
Further reading