This document provides an overview of information retrieval. It discusses what information retrieval is, which is finding documents relevant to an information need from a large document set. It compares information retrieval to databases, noting that information retrieval deals with unstructured and natural language queries over imprecise text matches, rather than structured data and defined queries used in databases. The document also outlines the basic architecture of a search engine, including the indexing and query processes, and covers techniques for indexing text like stopping, stemming, and handling synonyms. It discusses user interfaces and ranking algorithms used to order search results by estimated relevance.