Here on Flickr, you're in full control over the content licensing on your account.
Creative Commons licenses (CC) allow you to retain control over your images and give you more choices as to how they can be used. Check out their website for more information on the different types of Creative Commons licenses.
You can set a default license for future uploads in your account settings, or change the license on existing content.
Note: CC licenses are not revocable, so think carefully before choosing a CC license. For more information please see: Creative Commons - Frequently Asked Questions