Open source licenses differ mainly in what they require when code is redistributed. Permissive licenses like MIT and Apache 2.0 allow almost any use, including proprietary forks, as long as the copyright notice is kept. Copyleft licenses like GPL require derivative works to be released under the same terms. Apache 2.0 adds an explicit patent grant that MIT lacks. Projects should also consider contributor agreements, because accepting outside code without a license can create legal ambiguity about who owns the contribution.