An example of a good quality link exchange might be a when a CPA firm and Tax Law firm agree to funnel links back and forth, but not the same pages. Another might be where a foodie blogger who reviews food and your restaurant could also pair up. Some bad examples are where you sign up for someone else to manage your links and hope they pair you well. What if your gourmet chocolates gets paired with a plumber, or imagine what happens if your local Shoe Store blindly exchanges links and a shrewed SEO firm pairs you with their client a national Shoe Chain that sells your same stock at 10% less with free shipping, but buries your link so deep that no one will ever find it