A blogroll is a list of links to blogs that the blogger likes. A blogroll is usually included in the blog’s sidebar. Blogrolls are great organic traffic tools. With each blogroll that your blog is listed on readers can click on your link and visit your blog. Blogs with many incoming links (particularly those from high quality blogs as rated by Google page rank or Technorati authority), are usually ranked higher by search engines, which can bring additional traffic to your blog.
To be nice in the blogosphere if you get a link to your blog on a blogroll, you typically add their link in your blogroll. Of course, it’s entirely up to you to decide if a link to their site is appropriate.