Copyleft – How its different from Copyright
What is Copyright
A copyright is a collection of rights that automatically vest to someone who creates an original work of authorship – like a literary work, song, movie or software. These rights include the right to reproduce the work, to prepare derivative works, to distribute copies, and to perform and display the work publicly. [copyrightalliance.org]
Copyleft distinguishes from copyright.
Copyleft licenses exist within the legal structure of copyrights. Despite what the name implies, copyleft isn’t about abolishing copyrights. Rather, copyleft licenses are a subset of copyright licenses, and the goal is to restore freedom to users. Suppose a photographer releases a copyleft photo for anyone to use. As a user, you’re within rights to download that photo, modify it however you want, and then distribute it however you want to whoever you want—but you’d also have to permit anyone else to modify and distribute your work however they want. In other words, you might not be able to acquire a certain copyleft work without first paying for it. But once you do pay for it, you’re free to modify and distribute as long as you maintain the same copyleft freedoms in the derived work. The Linux kernel is licensed under the GNU General Public License (GPL), which is a copyleft license. Red Hat Enterprise Linux (RHEL) is a commercial operating system built on a modified Linux kernel. The desktop version of RHEL is sold for $49, but to abide by the GPL, the RHEL source code is included in the purchase. RHEL users are free to modify and redistribute the source code, which is how the free RHEL-clone operating system called CentOS came about. However, users aren’t allowed to resell RHEL itself because RHEL is protected by a trademark. – [makeuseof.com]
Copyleft is a copyright licensing scheme in which an author surrenders some, but not all, rights under copyright law. Instead of allowing a work to fall completely into the public domain (where no ownership of copyright is claimed), copyleft allows an author to impose some restrictions on those who want to engage in activities that would more usually be reserved by the copyright holder. Under copyleft, derived works may be produced provided they are released under the compatible copyleft scheme. – (From Wiki)