what is a digital library
According to Wikipedia at http://en.wikipedia.org/wiki/Digital_library, a digital library is “a library in which collections are stored in digital formats (as opposed to print, microform, or other media) and accessible by computers”. “The DELOS Digital Library Reference Model” mentioned in the Wikipedia article defines it as “[a]n organization, which might be virtual, that comprehensively collects, manages and preserves for the long term rich digital content, and offers to its user communities specialized functionality on that content, of measurable quality and according to codified policies”.
If you like to get more information about this topic, please see the Wikipedia article at http://en.wikipedia.org/wiki/Digital_library. There you can also find reference books that can be useful for you.