what is a digital library?

Answer 1

that depends
The college I went to for my undergrad used the term digital library for an online repository of software (opensource or bulk licensed) that had been made available for students and faculty to download and use
The college I now go to for my grad uses the same term for an online libarary that is created by digitizing/scanning the physical library in college. This sets up something like a whole repository of text data that you can search online. It calls for specialized Optical Character Recognition software to do this.

So you need to elaborate on what you are looking for