Different types of licenses

There are different categories of free software.

• Public domain software – the copyright has expired, the work was not copyrighted, or the author has released the software onto the public domain (in countries where this is possible).
• Permissive licenses, also called BSD-style because they are applied to much of the software distributed with the BSD operating systems. These licenses are also known as copyfree as they have no restrictions on distribution.
• Copyleft licenses, with the GNU General Public License being the most prominent. The author retains copyright and permits redistribution under the restriction that all such redistribution is licensed under the same license.