There are a lot of softwares and browser plugins that can download videos from youtube in different formats (mp3, mp4, flv, etc). The questions about the legality of the download and conversion of the videos is still being debating over the internet and usually the answers depends about if the downloaded content is copywriting or not. Some people says that it’s legal as long the video is not protected by the copyright laws.
By the other hand, when somebody uploads videos on youtube he/she has 2 options about the licence: standard youtube licence and Creative Commons Attribution license (reuse allowed). The default licence set when uploading any video is the standard youtube licence, so when there is a different situation the owner of the video must select the CCA licence.