Perfect Binding, commonly called "Soft Cover" or "Paperback" binding, is a binding method where the inside pages are stacked on top of each other, and the cover is wrapped around and glued in place. It is called Perfect Binding because the books are trimmed on the 3 non-spine edges after binding, producing a "perfect" finish as the pages all line up and the book has sharp square corners.
Other telltale features of this binding method are the square spine that is often printed on and the "hinges" on the front and back covers.
These hinges are created because the cover is not only glued on the face of the spine but on the front and back cover as well. The usual glue area is .25" from the edge of the spine to ensure the cover and pages stay secure.