Libraries are institutions where books are stored and preserved. Library catalogues are the key to these collections.
In many disciplines, especially in the humanities and social sciences, books are a major source of information – often more important than journal articles.
No library owns every book that has ever been published. To research a topic thoroughly, you may have to search more than one library's catalogue.
Union catalogues are databases that include the catalogues of many libraries.
Library catalogues list mainly books. However some library catalogues also list large numbers of maps, audiovisual items, music scores, manuscripts and other resources.
Many library catalogues now link to electronic resources. Some of those resources will only be available to registered users of that library, but other resources will be freely available.
Very few library catalogues list individual journal articles. The catalogue will tell you if the library has a particular journal, but normally it will not list all the articles in those journals.