Ontologies describe groupings of concepts that can be reused.
Some interesting principles on how to design ontologies: Towards Principles for the Design of Ontologies Used for Knowledge Sharing (1993)
Tools for creating ontologies
Protege seems to be the go-to tool for developing ontologies.
Existing RDF ontologies
A list of interesting ontologies/vocabularies.
A popular vocabulary for metadata (e.g.
Describes the things that Google understands.
Abstract ontology for workflows such as tasks and issues.
Provenance - the origin of things.
Ontologies for emergency information*
Linked Language Resources
Ontology for referring to languages proper (not only language tag of literal).
Developed as part of NEPOMUK.
The IBIS (bis) Vocabular
A vocabulary for Issue-based information system.