When I think about the library "catalog" I think that it will contain more than just metadata about library materials but also the materials themeselves. Think mostly books, journal articles, encyclopedia articles, definitions, images, data sets, etc. Moreover, the library "catalog" will enable people to do things with the items in the collection. It is more than just find and get.
This article is an introduction to the "brother and sister" Web Service protocols named Search/Retrieve Web Service (SRW) and Search/Retrieve URL Service (SRU) with an emphasis on the later.