Downloading from the web gives you access but not ownership! That means, while you can use any information, file or media within the scope of fair use, you cannot simply download and use material as if it were your own.
For media files, that particularly means you should not run software acting as a servier or file-sharing system (Kazza, Nabster, Limewire, or other bit torrents) or you risk copyright violation legal action.
according to Jason Mittell, http://justtv.wordpress.com/2010/07/27/expanded-fair-use-rights-for-everyone/, this revision provides uses for DVDs as well as exemption to documentary and noncommercial video.
Creative Common Licenses provide access to information in a shared environment. Materials still need to be cited and it certainly does not hurt to ask for permission if you want to adapt material.
Creative Commons also provides a good explaination about the public domain.