The Codevate blog has a tutorial posted by Chris Lush showing how to "drive user engagement" with Disquis in a Symfony-based application. He basically shows how to use commenting, with the help of Disquis, to encourage user participation with your content without having to build your own.
Engaging your audience is an ever-increasing concern when publishing content, since having a community that visitors can witness and interact with can help drive repeat traffic to your website. One such approach would be to let users write comments about your content, creating discussion that others can engage with. The temptation to implement your own comment system is strong since at a surface level it's a simple problem to solve, but to build a robust solution that can deal with moderation requirements such as banning & blocking users, editing comments, or just exploring related threads within a forum can easily turn your "users can write comments" requirement into another project in its own right.
Disqus to the rescue.
He starts by showing the benefits that using Disquis can provide to your site and outlines the steps required to get it up on your site. He then walks you through the process, step-by-step, of registering with the Disquis service, adding the code to your site and even integrating single sign-on with your own backend system.