Overview
The term "open source" describes practices in production and development that promote access to the end product's sources. Open source computer software is the most common and successful example of these practices.
The open-source software model allows a de-centralised community of programmers (a.k.a. "hackers") to collaborate on the development of a piece of software by making its source code (i.e. the underlying set of instructions that are translated into binary code and executed by a computer) open to everybody.
Complex development tasks are broken down into modular bits, which are worked on by specialist individuals or groups. Projects are often co-ordinated by a kind of "benevolent dictator", somebody with dictator-like powers over a project who is trusted by the community not to abuse those powers. The organisation of projects and the collaboration of developers is facilitated by the internet.
The resulting program is then made available under a General Public Licence that permits users to study, change and improve the software, and to redistribute it in modified or unmodified form.
By contrast, commercial software vendors, keep their source code secret in order to protect their ability to demand money for their proprietary software products.