Some authors/artists/creators give permission for others to use their work through licenses. The most popular licenses are through Creative Commons, which gives creators a vehicle to say, "Hell yeah, you can use my stuff, if you follow this license." Often the Creative Commons licenses call for attributing the work to the creator and using the work only in non-commercial activities or settings. For more on Creative Commons licensing, follow the link below.
Attribution
CC BY
This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation. This is the most accommodating of licenses offered. Recommended for maximum dissemination and use of licensed materials.
Attribution-ShareAlike
CC BY-SA
This license lets others remix, tweak, and build upon your work even for commercial purposes, as long as they credit you and license their new creations under the identical terms. This license is often compared to “copyleft” free and open source software licenses. All new works based on yours will carry the same license, so any derivatives will also allow commercial use. This is the license used by Wikipedia, and is recommended for materials that would benefit from incorporating content from Wikipedia and similarly licensed projects.
Attribution-NoDerivs
CC BY-ND
This license allows for redistribution, commercial and non-commercial, as long as it is passed along unchanged and in whole, with credit to you.
Attribution-NonCommercial
CC BY-NC
This license lets others remix, tweak, and build upon your work non-commercially, and although their new works must also acknowledge you and be non-commercial, they don’t have to license their derivative works on the same terms.
Attribution-NonCommercial-ShareAlike
CC BY-NC-SA
This license lets others remix, tweak, and build upon your work non-commercially, as long as they credit you and license their new creations under the identical terms.
Attribution-NonCommercial-NoDerivs
CC BY-NC-ND
This license is the most restrictive of our six main licenses, only allowing others to download your works and share them with others as long as they credit you, but they can’t change them in any way or use them commercially.
There are times when the material you would like to use is not covered by fair use, public domain, or previously granted permission (e.g., Creative Commons licenses), in which case you will need to ask permission of the copyright holder to use the material. The Columbia University Libraries/Information Services Copyright Advisory office has posted examples of permissions letters.