This document provides an introduction to indexing and defines several key concepts. It discusses what an information system and information retrieval are. Indexing is defined as the process of analyzing document content and expressing concepts in an indexing system using index entries. Different types of indexes are described, including author indexes and alphabetical subject indexes. The purposes of indexes are outlined as helping users locate needed information and providing related information. Levels of indexing from word indexes to computer-based indexes are also summarized.