Open source software is built on the principle of collaboration, transparency, and community involvement. The source code, which is the underlying code that runs the program, is made available for anyone to inspect, modify, and enhance. This model encourages developers to work together and improve the software continuously. Popular examples include the Linux operating system, the Apache web server, and the Mozilla Firefox browser. Additionally, open source can apply to hardware designs and even other forms of digital or non-digital projects, promoting innovation through shared resources.