The aim of Wikimedia Commons is to provide a media file repository "that makes available public domain and freely-licensed educational media content to all, and that acts as a common repository for the various projects of the Wikimedia Foundation."
The Commons is also multilingual. That is, multiple languages exist on the same page. This can be done since most "articles" are images with short descriptions.
Please see the project plan for full details.
Central media repository
Formerly each of the many Wikimedia projects—every single Wîkîpediya and Wîkîferheng, as well as Wikibooks, Wikisource, Wikinews, Wikiversity, and Wikiquote—used their own image namespace. There was no reliable way to reference images on other projects. Many good, free uploads on one wiki could go unnoticed on another. Thus, there was needless duplication of intellectual and creative work, as well as unnecessary redundancy and increased maintenance.
For the home page of this project, please see commons.wikimedia.org for more details.