You know what copyright is, but do you know what copyleft is?(short article)

Copyleft is a term referring to a license. It is used to indicate that a particular license allows to freely redistribute and modify the intellectual property(in most cases, source code) with the requirement of the same license to be preserved in the distributed works created from the original property. This type of license also preserves credit to the original owner. The redistributors must avail the source code of the redistribution, meaning that the redistributions need to be open source.

Copyleft software licenses are more protective in contrast to free software licences which do not require the license and credit to the original owner to be preserved.

It is in extreme contrast to Copyright licenses which do not allow the redistribution of the intellectual property. Popular copyleft licenses include GNU General Public License 3.0 and Mozilla Public License 2.0.