Open-source software (OSS) is computer software that is available in source code form and allows users to study, change, improve and distribute the software under a free license. Moodle is an example of open-source software that is used to create online courses. It was created by Martin Dougiamas and is based on social constructionist learning principles. Moodle has features like assignments, discussions, quizzes, and many plugins that can add new functionality. It also allows users to write their own modules to extend Moodle.