Licenses are permissions given by the copyright holder for their content. Licenses can be applied to copyrighted material in order to give permission for certain uses of the material. Copyright is still held by the creator in these cases, but the creator has decided to allow others to use their work. Sometimes licenses are purchased and sometimes they are given freely by the creator.
Licenses can be applied to allow reuse, redistribution, derivative works, and commercial use.
Creative Commons is the most frequently used and accessible free licensing scheme, but there are others that are used by certain communities. Licenses can also be applied by commercial entities that own copyright to an item such as a journal article. These licenses generally spell out limited usage for users and are available for a fee.
Creative Commons licenses are applied by the copyright owner to their own works. These are the most prominently used licenses of their type in the world. There are four components to the licenses that are arranged in six configurations:
The ND and SA components cannot be combined, as SA only applies to derivative works.
The six licenses (excluding CC-0 which is an equivalent to the Public Domain) are:
The following chart illustrates the permissions allowed by each license.
The GNU Public License is a free, copyleft license (see below) intended for software and can optionally be used for other works instead of the Creative Commons Licenses.
From their description:
The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things.
You can find more information about the GNU Public License here.
Copyleft, a play on the word "copyright," is the practice of offering users of a work the right to freely distribute and modify the original work, but only under the condition that the derivative works be licensed with the same rights. It is similar to the "Share Alike" stipulation of the Creative Commons licenses (and the SA icon resembles the copyleft icon). Copyleft licenses are often found on software packages, but can be used on any work.
Copyleft licenses give each person who possesses the work the same rights as the original author, including:
Freedom 0 – the freedom to use the work,
Freedom 1 – the freedom to study the work,
Freedom 2 – the freedom to copy and share the work with others,
Freedom 3 – the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.
In order for the work to be truly copyleft, the license also has to ensure that the author of a derived work can only distribute such works under the same or equivalent license.