Open-Source Software (OSS) is one of the most popular types. These are freely available programs with publicly available source code so that any user can change it. Open-source can even be contributory in nature.
While the terms open-source software and free software are often used interchangeably, there are slight differences between the two. The latter was a concept introduced around 1984 that sought to give value to the user’s freedom in terms of using programs. Software is considered free software when: it can be used for any purpose, can be studied to know how it works, copies can be distributed, and it can be improved by any user.