GPL-3.0 is a strong copyleft license. Anyone who distributes the software, modified or not, must make the complete corresponding source available under GPL-3.0. It includes an explicit patent grant and patent-retaliation clause.
It also includes anti-tivoization provisions — you cannot distribute GPL-3.0 software on hardware that prevents users from installing modified versions.
Pick GPL-3.0 when you want to make sure derivative works stay open. Pick AGPL-3.0 if you also want to close the loophole where software offered as a network service is not considered “distributed”.
How to apply
Add a file named LICENSE or COPYING with the full GPL-3.0 text. Each source file should start with a brief header identifying the file as part of a GPL-3.0 project:
// SPDX-License-Identifier: GPL-3.0-only
Full license text
GNU GENERAL PUBLIC LICENSE
Version 3, 29 June 2007
Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
Preamble
The GNU General Public License is a free, copyleft license for
software and other kinds of works.
The licenses for most software and other practical works are designed
to take away your freedom to share and change the works. By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program — to make sure it remains free
software for all its users.
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.
To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights. Therefore, you have
certain responsibilities if you distribute copies of the software, or if
you modify it: responsibilities to respect the freedom of others.
… this text is abbreviated. The full GPL-3.0 license is approximately
5,600 words and can be downloaded from
https://www.gnu.org/licenses/gpl-3.0.txt
TERMS AND CONDITIONS
0. Definitions.
1. Source Code.
2. Basic Permissions.
3. Protecting Users' Legal Rights From Anti-Circumvention Law.
4. Conveying Verbatim Copies.
5. Conveying Modified Source Versions.
6. Conveying Non-Source Forms.
7. Additional Terms.
8. Termination.
9. Acceptance Not Required for Having Copies.
10. Automatic Licensing of Downstream Recipients.
11. Patents.
12. No Surrender of Others' Freedom.
13. Use with the GNU Affero General Public License.
14. Revised Versions of this License.
15. Disclaimer of Warranty.
16. Limitation of Liability.
17. Interpretation of Sections 15 and 16.
END OF TERMS AND CONDITIONS
For the complete license text, see https://www.gnu.org/licenses/gpl-3.0.html
Raw text also available at /license-texts/GPL-3.0.txt — handy for curl.