It is part of the concept of Library 2.0 and is intended to supplement or even replace the existing OPAC catalogs.
[5] As the web became more widespread, library users developed "the expectation of being able to discover the collection in a search engine style".
These two features, search-engine-like interfaces and multidatabase search, began to appear in the same software systems.
"The discovery layer still uses the information and indexing in the integrated library system (ILS), but it also searches across proprietary databases and other electronic resources, all with the goal of revealing everything that a library owns or has licensed on a given topic be it a print monograph, an electronic journal article, streaming video, or a collection of archival documents"[7] The discovery layer can be looked on as the replacement for the OPAC.
Other discovery tools are free-standing software products: Blacklight and VuFind are open-source examples.