Open source describes software whose underlying foundation (its source code) is available to the public. All users can view, alter, and redistribute open-source software so long as they abide by the conditions set out by the license under which it is distributed. This type of software differs from proprietary software, such as Microsoft Office or Adobe Photoshop, whose source code is protected and modified only by the manufacturer's own developers. Popular open-source utilities include MySQL and WordPress.

Open-source software is generally protected by contracts called open-source licenses, which define the conditions under which one may modify and distribute the software. A condition of open-source software is that any user can freely examine, modify, and redistribute the program code; however, keep in mind that although some open-source software is also free of charge, many open-source projects charge fees for their use.

Read more about WordPress licensing in the WordPress codex. To learn more about open-source software and philosophy, visit the GNU Project website or review its Free Software Definition.