Institutional Repositories, Digital Libraries (muddiest points)

This may be a silly question but I figured I'd ask it anyway. Is there any difference between an institutional repository and a digital library? Or is a digital library an example of an institutional repository? If so what other kinds of institutional repositories are there?

Do institutional repositories and digital libraries keep everything they collect indefinitely? I am guessing they can simply add a new server to increase their capacity to store information but is appraisal done to identify and erase items whose rate of use may be low or non-existent?