What is the difference between permissive and copyleft licenses?

Permissive licenses (MIT, BSD, Apache 2.0) let you reuse the code in almost any project, including proprietary ones, as long as you keep the copyright and license notice. Copyleft licenses (GPL, AGPL, and weakly LGPL/MPL) require that derivative works are released under the same license, keeping the code open.

What does the Apache 2.0 patent grant add over MIT?

Apache 2.0 includes an explicit patent licence from contributors and a patent-retaliation clause: if you sue a contributor over patents in the work, your patent rights terminate. MIT has no express patent grant, relying only on an implied licence, so Apache 2.0 gives downstream users stronger patent certainty.

What makes the AGPL different from the GPL?

The AGPL closes the 'SaaS loophole'. Under the GPL, you only trigger source-disclosure obligations when you distribute the software. The AGPL also triggers them when users interact with the software over a network, so running modified AGPL code as a web service requires you to offer that modified source.

Are LGPL and MPL strong copyleft?

No, they are weak copyleft. The LGPL lets you link the library into proprietary software as long as the LGPL parts stay replaceable and their source is available. MPL 2.0 applies copyleft per file, so you must share changes to MPL-covered files but can combine them with proprietary code in other files.

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Open Source License Reference

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Comparing the major open-source licenses

Open-source licenses fall on a spectrum from permissive (do almost anything, just keep the notice) to strong copyleft (derivatives must stay open under the same terms). Choosing or complying with one comes down to three questions: can you use it in proprietary code, does it grant patents, and what must you disclose when you distribute. This reference compares the licenses you will meet most often.

How it works

Each license sets conditions that attach when you distribute (and, for AGPL, when you serve over a network). Permissive licenses — MIT, BSD, Apache 2.0 — only ask you to preserve the copyright and licence text; Apache 2.0 adds an express patent grant. Weak copyleft — LGPL (library linking) and MPL 2.0 (per-file) — keeps the licensed parts open while allowing proprietary combination. Strong copyleft — GPLv2, GPLv3, AGPLv3 — requires the whole derivative work to be released under the same licence, with AGPL extending that to networked use. GPLv3 and Apache 2.0 are compatible one way; GPLv2-only and Apache 2.0 are not.

Tips and notes

For maximum adoption, pick a permissive licence — MIT or Apache 2.0.
Prefer Apache 2.0 when you want an explicit patent grant and retaliation clause.
AGPL triggers source disclosure for network use — important for SaaS.
Keep the LICENSE file and copyright notices intact in every distribution.