Open sourcesoftware is software where the source code, the language programmers use to make computers do their jobs, is available to everyone. The opposite is closed source, where the source is not available to everyone. Open Source has been around for decades, but it became popular with Linux and BSD software communities. Anyone can change the code and see how it works. To protect the code, a special license is used. The most common kinds of licence are the GPL, BSD and LGPL. Wikipedia uses open source too.