Just for background reading...
The Creative Commons license is the one which grants all users permission to reuse your content as part of their own creation.http://www.youtube.com/t/creative_commonshttp://creativecommons.org/licenses/by/3.0/
(All the legal gibberish)
Creative Commons licenses provide a standard way for content creators to grant someone else permission to use their work. YouTube allows users to mark their videos with a Creative Commons CC BY license [attribution - reuse allowed]. These videos are then accessible to YouTube users for use in their own videos via the YouTube Video Editor. Attribution is automatic under the CC BY license, meaning that any video you create using Creative Commons content will automatically show the source videos' titles underneath the video player. You retain your copyright and other users get to reuse your work subject to the terms of the license.
The standard YouTube licensehttp://www.youtube.com/t/terms
Section 6.c (the important part):
For clarity, you retain all of your ownership rights in your Content. However, by submitting Content to YouTube, you hereby grant YouTube a worldwide, non-exclusive, royalty-free, sublicenseable and transferable license to use, reproduce, distribute, prepare derivative works of, display, and perform the Content in connection with the Service and YouTube's (and its successors' and affiliates') business, including without limitation for promoting and redistributing part or all of the Service (and derivative works thereof) in any media formats and through any media channels. You also hereby grant each user of the Service a non-exclusive license to access your Content through the Service, and to use, reproduce, distribute, display and perform such Content as permitted through the functionality of the Service and under these Terms of Service. The above licenses granted by you in video Content you submit to the Service terminate within a commercially reasonable time after you remove or delete your videos from the Service. You understand and agree, however, that YouTube may retain, but not display, distribute, or perform, server copies of your videos that have been removed or deleted. The above licenses granted by you in user comments you submit are perpetual and irrevocable.