Features
- Users were free to use software as they saw fit
- Free to study its source code
- Free to modify it for their own purposes
- Free to share it with others
- Doesn’t forbid companies from selling copies of software, as long as they allow their customers to share their code.
Caveats
- The GPL is a viral license, meaning that anyone who creates software based on code licensed under the GPL must also release that derivative code under a GPL license.