SKOS Simple Knowledge Organization System Reference
This document defines the Simple Knowledge Organization System (SKOS), a common data model for sharing and linking knowledge organization systems via the Web.
SPARQL 1.1 Query Language
This specification defines the syntax and semantics of the SPARQL query language for RDF.
RDF Schema 1.1
RDF Schema provides a data-modelling vocabulary for RDF data and is an extension of the basic RDF vocabulary.
RDF 1.1 Concepts and Abstract Syntax
RDF 1.1 Concepts and Abstract Syntax defines an abstract syntax (a data model) which serves to link all RDF-based languages and specifications.
OWL – Web Ontology Language
The W3C Web Ontology Language (OWL) is a Semantic Web language designed to represent rich and complex knowledge about things, groups of things, and relations between things.
Ontology Development 101: A Guide to Creating Your First Ontology
Still the best short practical introduction to building and maintaining an ontology: scope, classes, properties, reuse, and iteration. It is old, but still widely used and heavily cited.