Semantics – or, simply speaking, meaning – has always played an important part in information organization, processing, access and management. In the library world, tools have been designed that represent the semantic relationships among disciplines and their constituent concepts: classification schemes, subject heading lists, thesauri, etc. are all tools that in some way represent the semantic relationships among concepts, and information resources are mapped against such tools in order to process their semantic content to facilitate better organization and access. Although such tools have been successfully used for a long time, they have some inherent limitations that make them unsuitable for use in the web environment, especially as far as semantic information processing and management is concerned. Ontologies have been developed for this purpose; they are sophisticated information processing tools that allow computers to process information resources based on the meaning of their constituent parts. This chapter provides a general overview of ontologies from non-technical perspectives; many important references appear at the end that will lead readers to further information about different types of ontologies and their underlying technologies. The chapter begins with a definition of the origin and meaning of the term ontology, followed by a brief discussion of how it differs from other similar tools like taxonomy and thesauri. It then provides some common examples of ontology, and discusses the role played by an ontology in the organization and processing of electronic information. It also describes how to build an ontology, and discusses the characteristics of some ontology languages like OWL (Web Ontology Language). Several excellent resources on ontologies, and on ontology-building languages and techniques, are available, such as those on the W3C website referred to in the references.