Manual backlinks are those that are build or written by hand by a real person. Many faulty and spammy SEO companies use Spam software to shoot your link out onto the web. Of course, google and other major search engines are good at identifying these low-quality backlinks and almost immediately discount them as worthless. The worst part is that for a few days you may actually see a temporary benefit from those spam links but after a couple of weeks Google will find the links and mark them as spam causing your pagerank and traffic to spiral like a plane about to crash and burn. You will see your site go from page 1 in google to pages beyond 10. Spam links can ruin a webpage's authority status with the search engines and usually causes your site to plummet.