i can explain in more detail with pictures when i get home from work, but executive summary:
both trade paperbacks and mass market paperbacks are usually constructed via perfect binding, where you take a stack of loose-leaf sheets and dunk the spine edge in, basically, hot-melt glue (low-temp thermoplastic with a little flexibility to it). stick a cover on the outside of that bad bitch and you're done. very easy and cheap to manufacture, but not durable; not only does the soft cover provide no protection, pages can fall out individually if the glue fails for whatever reason. (i don't have a picture handy but just grab any mass market paperback off your bookshelf and look at the spine)
typically, or perhaps traditionally, when binding a hardcover ("case-bound") book you assemble the sheets into signatures, which are sewn to each other to form a text block, like so:
(well, admittedly, using both linen tape and french link stitch is sort of the belt-and-suspenders of textblock construction. in my defense though look at the fucking size of this tome) but the point is that even before you've gotten around to gluing anything, the textblock hangs together and functions as a book, albeit an unusually wobbly one -- so if the cover completely falls off or something, the rest of the book still hangs together.
the other method of construction i see on many mass-manufacture hardcovers and some trade paperbacks is that they've folded the signatures and sewn them individually (one at a time, not to each other) -- this is easy to do on a specialized sewing machine -- and *then* potted the spine in glue, like you do for perfect binding. this is less liable to lose pages if you fuck up the spine, because instead of each page being glued in individually, they're sewn together into signatures which provide more glue surface area apiece. (i can post a picture when i get home...)
uhh oh yeah endbands. endbands are the little decorative bits that get glued onto the textblock before it gets cased in -- this is in itself sort of a cheapo mass-manufacture imitation of more traditional sewn endbands, which actually provide some structural stability; modern glued-on endbands are really just decorative. here's a picture of a sewn endband on an example book from the bookbinding museum in sf (left), and a different textblock with endbands glued on (right). (the latter also has mull glued onto it, which is like... starched cheesecloth, kind of? you can use kozo paper here too; it also helps stabilize the spine for extra durability)
anyway on mass-manufacture hardcovers i often see really half-assed endbands that are glued on crooked or slightly undersized or something and i'm like "are you even TRYING" (they are not)
and also usually on recently manufactured books the entire case (the "hard cover" of a case-bound hardcover) is covered in paper, including the hinges, which is a terrible decision because the hinges are the part of the book that MOST needs the durability, being The Primary Moving Part. at least fucking cover the spine and hinges in bookcloth i beg. please. for me