Software is open source if its source code can be publicly downloaded by anyone. Free software means that anyone is able to download the source code and modify it in any way he wishes. He is also allowed to freely distribute the software as he sees fit. The word “free” in this context does not mean that the software is free of cost (which is nonetheless often the case), it means that the user is free to do anything with it that he wants.

There are several free and open source software licenses that formalise this concept, the most commonly used being the GPL. The most well known free software projects are Firefox, Linux and OpenOffice. There are many more, however.