It is the license used by the Linux kernel.
The GPL is the most popular and well-known example of the type of strong copyleft license that requires derived works to be available under the same copyleft.
Under this philosophy, the GPL is said to grant the recipients of a computer program the rights of the free software definition and uses copyleft to ensure the freedoms are preserved, even when the work is changed or added to.
This is in distinction to permissive free software licences, of which the BSD licenses are the standard examples.
Read the rest of the article here