What is a digital library? It may be obvious to you, but not to your patrons (also called users or customers). In fact, there is not a standard definition. The School of Information Resources & Library Science at the University of Arizona defines it as a:
Collection of texts, images, etc., encoded so as to be stored, retrieved, and read by computer.
That's not a user-friendly definition, so yours will need to be something simpler and appropriate. Whatever your definition is, make sure that you communicate it to your users and include examples so they can see (understand) what you means. Try to create a definition that is jargon-free. Teach the definition to your staff and include it on materials your produce about your digital library and on the digital library's web site.
Think it's intuitive what's on your digital library web site? Think again.
Even though you may be intimately familiar with the materials on the digital library web site, your users may not be. Help them by providing explanations about the materials. Give overviews of the databases, the image collections, and other materials storied there in electronic form.
Think your users know how these materials relate to what is in your physical brick-n-mortar library (if you have one)? Probably not. To be sure, include that information online. Also post information in your physical library that tells your users what complementary resources are online.
Tell them "the obvious" because it is not obvious. They'll thank you for it.