Perfect binding is a widely used soft cover book binding method, the pages and cover are glued together at the spine. The fundamental difference between perfect and PUR binding is the adhesive used in the process, PUR adhesive gives superior adhesion over perfect binding.
Thread sewn is similar to perfect/PUR binding. Thread sewn employs multiple threads to sew the signatures together. The thread appears at the middle at the middle of a signature and the books are much more durable and lay flatter.