Most people have heard of Linux and Open Source Software and that Linux is distributed under an Open Source License. Many people think that all Open Source Software costs $0. Nothing could be further from the truth. While most Open Source Software, including Linux, does cost $0, this is not the "essence" of Open Source Software. The key thing about an Open Source License is that the source code (blueprints) for the software must be made available. There are other things that define Open Source Software but none of them refer to the monetary cost.
Chapter 2 of my book on Linux discusses Open Source Software. Here are the first 57 pages of the book. They contain Chapter 2 and other material. This is copyrighted material.
Here is the original undedited copy of my 2010 NEA article. This is copyrighted material.