This short video explains how the Creative Commons License works.

This video is from Lawrece Lessig's YouTube page and is CC-BY (2.5) licensed.

Here are the different kinds of CC licenses:

Public Domain (PD)

By licensing your works as Public Domain, you release them from copyright completely. Anyone can use your works in their own projects without giving you credit.

Attribution (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, in terms of what others can do with your works licensed under Attribution.

Attribution, Non-Commercial (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, Non-Commercial, Share-Alike(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. Others can download and redistribute your work just like the by-nc-nd license, but they can also translate, make remixes, and produce new stories based on your work. All new work based on yours will carry the same license, so any derivatives will also be non-commercial in nature.