Creative Commons is a free-to-use copyright/licensing framework.
People use Creative Commons when they publish* something like a document, a blog, an image or a video, and want to share it and let others re-use it. Creative Commons is mainly used for items made available on the web, but it can apply to printed materials too.
There are six Creative Commons licences to choose from, ranging from the permissive to the restrictive. Below is an example of the icon for one of the six licences: click on the icon and a new window will open showing the the licence deed. The document below entitled Six Licences for Sharing Your Work explains the differences between the six licences.
Creative Commons also promotes public domain tools such as CC0 (CC zero) and the public domain mark. If something is CC0 it means the rights holder has waived all copyright rights and you are free to use the item as you wish, without even crediting the author/creator. Public domain marks are applied to items free of known copyright restrictions. The document below entitled Guide to Using Public Domain Tools explains more about this.
*Publish = make something available to the public in any format.