This document provides an introduction to ontology from a lecture on the topic. It discusses the need for shared semantics and meaning mediation when integrating different information systems. Ontologies can provide formal specifications of conceptualizations to enable semantic integration and interoperability. Examples are given of how ontologies can be used for applications like open information systems, the semantic web, e-commerce, and more. Key points covered include what an ontology is, levels of ontological precision, and how ontologies differ from things like XML schemas, standard vocabularies, and conceptual data schemas in providing formalized meanings.