The keys are the MARC21 or EAD metadata and the large amount in number and size of images.
The QURL solution
MARC21 is a highly customizable and very extended format for describing metadata of monographs (books). To prevent overkill for the common user, the metadata is consolidated into a a more useable format. To prevent long waiting times for the images to download, but to keep a high quality to be able to actual read the image the images are converted to DjVu format.