What is WikiData?

Wikidata is a repository of statements, used by Wikipedias in all languages. Wikipedia is huge. The English Wikipedia has over 5 million articles [1]. However, Wikidata has over 400 million statements [2].
This is what Wikidata knows about Douglas Adams:
https://www.wikidata.org/wiki/Q42
The raw RDF is at
https://www.wikidata.org/wiki/Special:EntityData/Q42.rdf