GPL, or General Public License (GPL), refers to a free, copyleft license for software. It grants anyone the rights to freely use, read, copy, share, modify, and distribute a computer program or other kinds of work. GPL was originally written for the GNU Project and was the first copyleft license to be adopted for general use. It was created in early 1989 by Richard Stallman, the founder of the Free Software Foundation (FSF).
As opposed to copyright, the term copyleft means that the GPL allows for derivative works to be published, but require them to be distributed under the same license terms as the original work. Thus, users can’t use GPL-licensed software and release a derivative work under another type of license. This is not the case for other types of free software licenses, such as the Berkley Software Distribution (BSD) and the MIT licenses.
The BSD and MIT licenses fall within the category of permissive licenses. While both copyleft and permissive licenses allow users to copy, change, and distribute software, their conditions are somewhat different.
On the one hand, copyleft licenses guarantee that open-source software remains available to everyone. It also avoids someone else to profit from a piece of work that was made available for free. Copyleft advocates tend to be more concerned in retaining some control over their work.
On the other hand, permissive licenses allow software to be used widely, as long as the original developers are referenced and attributed for their work. In other words, a permissive license allows anyone to copy, change, and distribute a piece of work, under any kind of license. The only requirement is to give credits to the original creators.