What Are Public Domain Photos?
Public domain images are completely free, copyright-free, and royalty-free. Public domain photos have fewer limitations than most stock photos, stock images, and stock photography. Because there is no copyright on these photos and images, you can use most of them for commercial purpose with few limitations. But that there may still be limitations based onpublicity or privacy or trademark laws. And remember that it is still courteous to give credit to the photographer or illustrator.
reference：
署名
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.
署名-相同方式共享
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.
一些中文解释：
公有领域（英语：Public Domain）是人类的一部分作品与一部分知识的总汇，可以包括文章、艺术品、音乐、科学、发明等等。对于领域内的知识财产，任何个人或团体都不具所有权益（所有权益通常由版权或专利体现）。这些知识发明属于公有文化遗产，任何人可以不受限制地使用和加工它们（此处不考虑有关安全、出口等的法律）。
创立版权制度的初衷是：借由给予创作者一段时期的专有权利，作为（经济）刺激，以鼓励作者从事创作。当专有权利期间届止，作品便进入公有领域。公有领域的作品由于没有专属权利人，因此公众有权自由使用它们。
http://www.hudong.com/wiki/Creative+Commons