youtube-dl is a handy little command-line utility that, with the right command, automagically downloads videos from Youtube as well as other platforms such as Vimeo, Lynda.com, BBC, CNN etc..(Full list of supported websites)
Kindly proceed to youtube-dl's Github repo for detailed installation instructions for your respective OS
In case of an error, make sure you have Python 2.6, 2.7 or 3.2+ installed as youtube-dl needs it to run.