In a hardcover book, endsheets (also known as endpapers) are the sheets of paper that are glued to the inside covers. Endsheets add durability and support to the binding, helping to keep the book intact. They are left blank and can't be customized.
The flyleaf is a part of the endsheet that is not glued to the cover. It is the first free page you see when you open the book's cover. The flyleaf is left blank and can't be customized. In the finished product, it can be used for personal hand-written inscriptions and gift notes. The flyleaf serves as a protective barrier for the text block, preventing damage to the first and last pages of the book's content.
All of BookBildr's partner printing facilities use endsheets, but not all use an additional flyleaf (extra blank page) at the beginning and end of the book. Thus, hardcovers printed at different printing facilities may have or may not have flyleafs.
Having a flyleaf vs. no flyleaf varies in appearance only, and both methods are equally secure in effectively binding the book block to the hardcover board.