Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone, in their own language. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted here. The repository is created and maintained not by paid archivists, but by volunteers. The scope of Commons is set out on the project scope pages.
Wikimedia Commons uses the same wiki-technology as Wikipedia and everyone can edit it. Unlike media files uploaded to other projects, files uploaded to Wikimedia Commons can be embedded on pages of all Wikimedia projects without the need to separately upload them there.
Unlike traditional media repositories, Wikimedia Commons is free. Everyone is allowed to copy, use and modify any files here freely as long as they follow the terms specified by the author; this often means crediting the source and author(s) appropriately and releasing copies/improvements under the same freedom to others. The license conditions of each individual media file can be found on their description page. The Wikimedia Commons database itself and the texts in it are licensed under the Creative Commons Attribution/Share-Alike License. More information on re-use can be found at Commons:Reusing content outside Wikimedia and Commons:First steps/Reuse.
You can improve Wikimedia Commons most if you contribute what you can do best:
Contribute your work
If you're a good photographer, don't hesitate to contribute your valuable images. If you're a good designer, look which diagrams and animations are badly needed. If you're a good musician or film/theater artist, do not hesitate to publish your own records of your performances of free pieces here.
Contribute your skills
However you do not need to upload your own files. There is plenty of other very important work to do:
In order to be able to upload files on Wikimedia Commons, you need to be logged in. You can register at the "Log in / create account" link in the upper right corner and enter a user name that will be used at all of your uploads/edits on images and texts. However if you just want to edit pages you don't need to be logged in (although it is encouraged). If you have taken advantage of unified login, then you are already signed up at Commons.
First steps tutorial
Our first steps help file and our FAQ will help you a lot after registration. They explain how to customize the interface (for example the language), how to upload files and our basic licensing policy. You don't need technical skills in order to contribute here. Be bold contributing here and assume good faith for the intentions of others. This is a wiki - it is really easy.