A spiral-bound book or notebook is held together by a wire (or plastic) that is coiled round and round like a spring, threaded through a row of small holes punched down one edge of the pages. You have certainly used one — most cheap notebooks are made this way. The reason people like them is practical, and that is usually why the word is used: a spiral-bound book lies completely flat when open, and you can fold the cover all the way back so the book takes up only half the space on a desk. That makes it the usual choice for notebooks, sketchbooks, cookbooks, wall calendars, and manuals meant to be propped open while you work with your hands. The word is nearly always an adjective before a noun, and it is the ordinary word for it in shops and printing — if you order printing, 'spiral-bound' is one of the choices you tick. The downside people mention is that the wire bends out of shape in a bag and pages tear free.