Tracker is an RDF (resource data format) storage engine. RDF consists of triplets such as subject:verb:object. For example, there could be standard triplets for book titles and authors, such as Othello:has-author:William Shakespeare. A standardized set of triplet forms is called an ontology.
Tracker provides a storage for such triplets, and a query engine in the form of the SPARQL query language.
GNOME uses Tracker as a storage for document metadata. A document's metadata may include its title, authors, copyright, modification date, and keywords. All this metadata is stored as RDF triplets in Tracker, and queried with SPARQL by applications such as GNOME Documents. The ontology used is based on various standard sub-ontologies: Dublin Core for document metadata, and NEPOMUK for annotations, files, contacts and other items.
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