Open source software is a movement that started in the software industry in the 1980s. Its origins are up for debate, but most believe that the concept came about in 1983 when a company called Netscape released its Navigator Web browser source code to the public, making it freely available to anyone who wanted to dig through it, modify it, or redistribute it.
WordPress software users need a basic understanding of the open source concept and the licensing upon which WordPress is built because WordPress's open source policies affect you as a user — and greatly affect you if you plan to develop plugins or themes for the WordPress platforms. A basic understanding helps you conduct your practices in accordance with the license at the heart of the WordPress platform.
In this chapter, we introduce you to open source, the Open Source Initiative, and the GPL (General Public License), which is the specific license that WordPress is built upon (GPLv2, to be exact). You also discover how the GPL license applies to any projects you may release (if you are a developer of plugins or themes) that depend on the WordPress software and how you can avoid potential problems by abiding by the GPL as it applies to WordPress.
IANAL — I Am Not a Lawyer — is an acronym you find in articles about WordPress and the GPL. I use it here because I am not a lawyer and the information found in this chapter shouldn't be construed as legal advice. Rather, you should consider the chapter an introduction to the concepts of open source and the GPL. The information presented here is meant to inform and introduce you to the concepts as they relate to the WordPress platform.