Creative Commons (CC) is a non-profit organisation, founded in the United States in 2001, that publishes free, ready-made licences creators can attach to their work; the first set appeared in December 2002. The licences do not replace copyright but rely on it, granting the public permissions that would otherwise have to be requested.
There are six main licences, built from four elements: BY (credit the creator), SA (share adaptations under the same terms), NC (non-commercial use only) and ND (no adaptations). They range from CC BY, the most permissive, to CC BY-NC-ND, the most restrictive; the current version, 4.0, was published in November 2013. CC0 is separate: a dedication that places a work as close to the public domain as the law allows. A CC licence cannot be revoked for copies already shared, unless the user breaches its terms.
For musicians, two points stand out. NC music cannot be used commercially without separate permission from the rights holder, and membership of some collecting societies can limit an author’s ability to use CC licences at all. Specific uses are worth checking with the licensor or a professional.