What we mean when we talk about discovery services:
The idea behind a Web Scale Discovery Service is to give your library users a single point of entry to your collection and its physical and digital content. There are layers to this sort of service which often include a central index and a link resolver. The index holds information (metadata including author name, issue number, volume, etc) about your collection while the link resolver takes the user directly to the content they're searching for. Generally, discovery services require regular upkeep and re-configuration as you grow or trim your collection.