Copyleft is the practice of using copyright law as a means to get rid of any restriction on distributing materials and modifying others' works. In other words, under this practice, you are free to copy, modify, and/or redistribute materials in any way you like, provided that you redistribute it according to the Creative Commons Attribution License (CCAL).

So when you reuse copies, you have to acknowledge the source and indicate that the material is copyleft by giving a link to the CCAL. This informs other people that they can copy, modify, and/or redistribute materials, but they must use similar licensing conditions.

Freedom

Codifying the copying terms for a licensed material is a general practice for using copyleft. Typically, any such license gives every person who has a copy of the material the same freedoms enjoyed by the author. These freedoms include using and studying the work, copying and sharing the work with others, modifying the work, and distributing modified (derivative) works.

The freedoms mentioned above do not guarantee that a modified material will be distributed under similar liberal terms. For the material to be actually copyleft, the author of a modified work must be required to distribute the work under equivalent or the same license.

Strong and weak copyleft licenses

In "strong copyleft" licenses, provisions are efficiently imposed on any kind of modified works. On the other hand, in "weak copyleft" licenses, not all modified materials contain the copyleft license. The GNU GPL is the most popular free software license, which uses strong copyleft. Free software licenses that use "weak" copyleft include the Mozilla Public License and the GNU LGPL.

Full and partial copyleft licenses

In a "full copyleft" license, all parts of a material (excluding the license itself) may only be used under the terms of its copyleft license. "Partial copyleft" licenses, on the other hand, exclude some parts of the material from the provisions of the copyleft, therefore allowing distribution of some derivations under terms excluding the copyleft license.

Share-alike copyleft licenses

"Share-alike copyleft" licenses require that any freedom granted about the original material must also be granted on compatible or exactly the same terms in any modified work. This suggests that all copyleft licenses are automatically share-alike licenses, but not vice versa, as many share-alike licenses have further restrictions, such as prohibiting commercial use. Examples of this type of copyleft are some permutations of the CCAL.