Open Source is a term used to denote any software that is developed collaboratively and made available publicly. The mantra behind open source is that everyone is aware of what is being built so that knowledge is spread across the community and security becomes a shared responsibility. The majority of the internet runs on open source software. Most popular web servers that are hosting web applications are using open source software under the hood. Open source is free to use but illegal to sell. Many companies take a version of open source software and commercialize it by providing additional technical support.
in other words
The Wikipedia model: everyone can contribute to make stuff better.