ATrackBack is a mechanism used for communication between blogs: if a blogger writes a new entry commenting on, or referring to, an entry found at another blog, and both blogging tools support the TrackBack protocol, then the commenting blogger can notify the other blog with a "TrackBack Ping"; the receiving blog will typically display summaries of, and links to, all the commenting entries below the original entry. This allows for conversations spanning several blogs that readers can easily follow. – Information quoted from the WIKI
If you write a post that links to another blog article, and both blogs support trackbacks, your article link will appear as a comment on the post of the other blog that you are linking to.
Why use trackbacks?
Trackbacks are an automatic way to promote your articles and post comments on other blogs. They offer a connection of related content between blogs, and help promote participating blog authors content.
Enabling WordPress trackback support:
Login to your WordPress Admin Panel and navigate to the Options -> discussion section.
Check the option toAllow link notifications from other Weblogs (pingbacks and trackbacks)
Click Update Options
How to Trackback:
Using Trackbacks in WordPress is easy. When writing a post, simply copy the permalink of the post you want to trackback to into the trackback section.
Optionally, you can simply link to a permalink within the content of your posts and a trackback will be achieved.