An ILS is a single system that automates the business conducted by a library. Typically, it covers at least three areas: circulation, patron access and acquisitions.
The circulation module is the backbone of the system. It allows librarians to check books in and out. It handles fines, requests, holds and all of the related bits.
Patron access, or On-line Public Access Catalog (OPAC), is the modern equivalent of a card catalog. This is where library users go to look up books, perform searches, check their account status and the like.
The acquisition module allows the library to plan for growth, make purchases and accept donations. Librarians might place orders through this system, and they will certainly receive books with it as they arrive.
Another feature that an ILS might support is “weeding”. This is the mechanism that libraries use to control their collection, carefully selecting books that need to be removed in order to make room for new acquisitions.