The Creative Commons is an organization that provides an easy way for creators to preemptively give permission to use their work under a certain set of conditions, such as giving credit to the creator, or requiring that you also give the same sharing permission on your work. The Creative Commons website provides detailed information, tools, and icons for a creator to use with their works, however a simplified explanation of each type is provided here.
The BY is used to indicate that attribution is needed. You must give credit by providing the name of the creator(s), title of the work, a link to the license, a disclaimer notice, a link to the original material, and note if any changes were made.
SA stands for "Share Alike". In addition to everything needed for the CC BY, if you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.
NC stands for "Non-Commercial". In addition to everything needed for the CC BY, an ND indicates that it can not be used in a way that would provide monetary compensation to the one using the work.
ND stands for "No Derivatives". In addition to everything needed for the CC BY, you can not distribute the work if you remix, transform, or build upon the original.
Works under this license need attribution, can not be used commercially, and any modifications or adaptations have to be shared with the same license.
Works under this license need attribution, can not be used commercially, and can not be modified.
You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. The creator has waived their rights, and essentially donated it to the public domain.