What is the Commons?
The Commons prevents duplication of uploaded media by having one place that media can be uploaded to for use by all WMF projects. For example, one image uploaded to the Commons can be embedded on pages in every language Wikipedia.
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.
Formerly each of the many Wikimedia projects—every single Wikipedia and Wiktionary, as well as Wikibooks, Wikisource, Wikinews, Wikiversity, and Wikiquote—used their own image namespace. There is 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 home page of this project, please see commons.wikimedia.org for more details.