Finding documents, or the information contained in them, in a library or other collection, selectively recalling recorded information. Methods of retrieval vary from a simple index or catalogue to the documents, to a computer-based system. Classification, indexing and machine searching are all systems of information retrieval. Harrod's Librarian's Glossary .
Search engines are programs written to query and retrieve information stored in a database.
Search engines differ from database to database and depend on the information stored in the database itself. Habits or paradigms of information retrieval. Library resources, such as catalogs, indexes, and databases, share certain characteristics. Selection criteria are usually evaluative. Generally assigns subject headings or descriptors and may include abstracts written by information professionals.