Yesterday's post mentioned the CC-BY license that covers most of the images that we use on the blog and elsewhere.
Creative Commons is a non-profit organization that developed and promotes a family of licenses to provide an easy way for creators to specify what others may or may not do with a creative work. Each license has a simple abbreviation that adds a specific constraint:
Some combinations: "cc by", "cc by-nc", "cc by-nd", "cc by-sa", "cc by-nc-sa", "cc by-nc-nd".
The set of options is generally well thought out. However, there's also a potential source of confusion. Merely saying that a work is covered under a Creative Commons license says very little. The details are critical, and it's vital to check which cc license before proceeding. It would be helpful if the organization played a stronger role in encouraging people to post the exact license rather than just using the umbrella term.
There's also one useful license option that is missing (or at least unclear): requiring that any changes to the actual work are shared, but allowing the original or derivative work to be incorporated into a larger work without forcing the larger work into a certain license. (Model: the LGPL for software.)
A tidbit: in 2009, Wikipedia migrated from the GFDL to cc by-sa.