Creative Commons is a form of licensing for media, such as artwork, photos, videos, music, and writing. It's standardized, has concise labels, well understood around the world, and makes your work more searchable and filterable.
When you're making things based on other people's creative work, check for a Creative Commons license! That way, you'll know exactly whether you're allowed to do things like make covers or remixes, draw fanart based on existing designs, and if you can repost content.
Creative Commons does NOT always mean public domain. There are several options for CC licenses, and you can choose the ones that best fit what you want for your own original content. For more detail about what the licenses mean, and official documentation, please see the official Creative Commons website.
CC0 - Public domain. All of your copyright claim is given up, and your work can be used in any way by the public.
BY - Attribution. The work and any derivatives must be credited to the original creator. Creative Commons recommends that credits contain the title, author, source, and license. (Anyone who wants to use your work without credit can still ask for your permission.)
NC - Non Commercial. The work can only be used for non-commercial purposes (Anyone who wants to use your work commercially can still ask for your permission.)
ND - No Derivatives. The work can only be shared and distributed exactly as it is, with no modifications or derivatives whatsoever. (Anyone who wants to make derivatives of your work can still ask for your permission)
SA - Share Alike. Any derivatives of your work must use the same Creative Commons license as well.
These options can also be combined.
CC BY: The work can be shared and changed in any way, commercially or non commercially, even if they put different licensing terms on the derivative, as long as they give credit for the original.
CC BY-SA: The work can be shared and changed in any way, commercially or non commercially, as long as they use the same CC-BY-SA license on derivatives and give credit for the original.
CC BY-ND: The work can be shared in any way, commercially or non commercially, as long as they do not change the work at all and give credit.
CC BY-NC: The work can be shared and changed in any way, as long as they do not use it commercially and give credit for the original.
(I highly recommend this license for people in the vocalsynth community!)
CC BY-NC-SA: The work can be shared and changed in any way, as long as they do not use it commercially, use the same CC BY-NC-SA license on derivatives, and give credit for the original.
(This is the license that I use for my original songs. It gives people permission to make covers and remixes of my songs, as long as they aren't earning any money and also allow further derivatives. For example, if someone else wants to make a video on top of it, or use it as background music in other projects.)
CC BY-NC-ND: The work can be shared in any way non-commercially, with no changes to the original at all, and with credit to the original creator.