Creative Commons (CC) licenses were created in hopes of creating alternatives to the "All rights reserved" default of traditional copyright. Content creators can choose from a variety of options to generate a license that best suits the level of "open" that they'd like to commit to. These licenses are generally composed of the following base elements:
Attribution (abbreviated as "by") This is a requirement of all CC licenses in that it establishes that any use of your work must contain an attribution to its source. This does does indicate that you, as original creator, necessarily endorse or support the subsequent work, however.
ShareAlike (sa) This gives others permission to use, copy, distribute, and modify your work as long as they distribute the new work with the same terms.
NonCommercial (nc) This gives others permission to use, copy, distribute, and modify your work as long as they do not do so for commercial purposes.
NoDerivatives (nd) This gives others permission to use, copy, distribute, original copies of your work, and prohibits modification of your work.
Given these, you can now generate a CC license based on a combination of the above elements.
For example: A CC-BY-NC license means that your license allows for the use of your work as long as a proper attribution is attached, and that it is used for non-commercial purposes.
Another option, and the most generous, is the CC0 license. This license is used to give creators the ability to waive their copyright interests in whichever jurisdictions they have rights.