The easiest way to download YouTube videos is to use youtube-dl.
To install it using Debian based distribution like Ubuntu use command:
$ sudo apt-get install youtube-dl
Example usage:
$ youtube-dl http://www.youtube.com/watch?v=O5D1O7eb_H4
[youtube] Setting language
[youtube] O5D1O7eb_H4: Downloading video webpage
[youtube] O5D1O7eb_H4: Downloading video info webpage
[youtube] O5D1O7eb_H4: Extracting video information
[download] Destination: O5D1O7eb_H4.flv
[download] 100.0% of 124.37M at 87.78k/s ETA 00:00
