By default, everything you create is under exclusive copyright. Without a license, nobody can legally copy, distribute, or modify your work — even if it's publicly visible on GitHub. Choosing an open-source license is a declaration that you want your work to benefit everyone.
“Free software” means free as in libre (freedom), not gratis (no cost). The four freedoms:
| License | Type | Key Property |
|---|---|---|
| MIT | Permissive | Anyone can use for anything; only attribution required |
| BSD | Permissive | Similar to MIT; multiple variants |
| Apache 2.0 | Permissive | Adds explicit patent grant |
| GPL v3 | Copyleft | Derivatives must use same license if distributed |
| AGPL v3 | Strong Copyleft | Derivatives must use same license even if only run as a service |
| CC0 | Public Domain | All rights waived |
The Sony/FreeBSD example: Sony took FreeBSD 9.0 (permissive BSD license) and built the PlayStation 4's Orbis OS on top of it. Decades of community work absorbed into a proprietary commercial product with no obligation to give anything back. Permissive licenses allow this.
Copyleft uses copyright law to ensure a work remains free. Instead of “all rights reserved,” it's “these freedoms must be preserved.”
If you modify copyleft-licensed work and distribute it, you must release your modifications under the same license. The code and its freedoms become legally inseparable — a “viral” effect that prevents enclosure.
GPL requires sharing source code only when you distribute software. In the SaaS era, companies can modify GPL code, run it on their servers, and let users access it over the internet without ever distributing it — so they never have to share their changes.
AGPL v3 closes this: if users interact with your software over a network, you must make the source available. Corporate SaaS users of AGPL code must contribute their modifications back.
Recommendation: Use AGPL v3 for software you want to remain free in the SaaS world.