It has become more common for the copyright holder themselves to license their work to make it useable for others in different ways. Creative Commons is a licensing tool used to make it easier for people to share their work (in its entirety, or in parts). The copyright holder can choose which conditions apply for other people wanting to use their work through a simple system of licensing conditions. If you find material with a Creative Commons licence you can easily see how you are allowed to use that material, and you do not have to ask for specific permission to use it. Don't forget to always reference though!

Creative commons consist of four different licences which can be combined in different ways. Common for all of these licences is that you always have to credit the copyright holder if you use their work. The most lenient license, CC BY, allows you to share, change, and even use the work for commercial purposes. The licences can be combined in different ways.

BY = Attribution (credit always have to be given)

BY+NC = Attribution + NonCommercial

BY+NC+ND = Attribution + NonCommercial + NoDerivatives (no changes)

BY+NC+SA = Attribution + NonCommerical + NoDerivatives + ShareAlike (if you use the material in you work you have to share it under the same licence)