Open Source Software is computer software (a program) that is created within a collaborative, community-driven environment. The source code that makes up the software is available to the public, and the developers and users of the software are encouraged to give back to the project with the goal of creating a better product for everyone to enjoy. Where a corporation may have a small department working to improve bugs and useability, open source software is available to the public and, depending on the popularity of the software, there could be thousands of people working to improve the software in some fashion. Open source projects are most often free and are funded by donations and volunteers.
WordPress and Mozilla Firefox are two highly popular examples of open source software.
For a more detailed definition, see The Open Source Initiative website (opens in a new tab).