Comparison: Open Source Licenses

Copyleft (a play on the word copyright) is the key factor that differentiates the various open software licenses. Copyleft is a license requirement that governs the distribution of modifications to the original open source software. If a license contains a strong copyleft provision, anyone who modifies the source code and distributes it to the public must license the modifications back to the public under the same terms as the original software. This means that you must give up private ownership of any changes you make to copyleft software, unless you elect not to make the modified software available publicly.