Permissive and copyleft licences

Licences come in two families. Permissive licences, such as MIT, BSD and Apache 2.0, let anyone use the code, even inside closed-source products (software whose code is kept private). The main condition is keeping the copyright notice.

Apache 2.0 also adds an explicit patent grant, a written permission to use any patents the contributors hold on that code. Copyleft licences, such as the GPL, work the other way. If you distribute software built from the code, you must release it under the same licence.

The AGPL extends that rule to software offered to users over a network, like a web app. LGPL and MPL are weaker copyleft: the rule covers only the library, or only the files that were changed, not the whole program. This matters when you pick projects, when companies decide what they can use, and later when you license your own work.