Debian is an Operating System developed on Linux platform, and as such is completely freeware. Debian.
org is the official site for this project, and in it individual users or professional developers and programmers will be able to find all the necessary resources in order to get this O.S. going and to improve it: Descriptions (Social Contract, partners, donations), news, sources, packages, documents, developer corner. The site provides thorough support and tutorials that should probably be best used by pros of all around the world, as Debian is supported in most western languages and several Asian ones as well. 
