Open source software refers to computer programs where the source code is available to the public, allowing users to study, change and improve the software. It is often developed collaboratively. Open source software has advantages like better quality, reliability, flexibility and lower costs compared to proprietary software. The peer review process also helps improve code quality as any issues can be quickly identified and fixed by the community. Popular open source programs that power much of the internet include the Linux operating system, Apache web server, Firefox browser, and content management systems like Drupal and WordPress.