They are an integral part of bibliographic control, which is the function by which libraries collect, organize and disseminate documents.
A popular form of keywords on the web are tags, which are directly visible and can be assigned by non-experts.
The Simple Knowledge Organization System language (SKOS) provides a way to express index terms with Resource Description Framework for use in the context of the Semantic Web.
[1] Many journals and databases provide access to index terms made by authors of the respective articles.
The quality of these two types of index terms is of research interest, particularly in relation to information retrieval.