To download a video, you type the URL after the command like so:

youtube-dl 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

To select the video quality, first use the -F option to list the available formats, here’s an example,

youtube-dl -F 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

Here’s the output:

[youtube] Setting language

[youtube] P9pzm5b6FFY: Downloading webpage

[youtube] P9pzm5b6FFY: Downloading video info webpage

[youtube] P9pzm5b6FFY: Extracting video information

[info] Available formats for P9pzm5b6FFY:

format code extension resolution  note 

140         m4a       audio only  DASH audio , audio@128k (worst)

160         mp4       144p        DASH video , video only

133         mp4       240p        DASH video , video only

134         mp4       360p        DASH video , video only

135         mp4       480p        DASH video , video only

136         mp4       720p        DASH video , video only

17          3gp       176x144     

36          3gp       320x240     

5           flv       400x240     

43          webm      640x360     

18          mp4       640x360     

22          mp4       1280x720    (best)

The best quality is 22 so use -f 22 instead of -F to download the MP4 video with 1280x720 resolution like this:

youtube-dl -f 22 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

Or optionally use the following flags to automatically download the best audio and video tracks that are available as a single file:

youtube-dl -f best 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

If you encounter any error during the muxing process or an issue with the video quality selection, you can use one of the following commands:

youtube-dl -f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/bestvideo+bestaudio' --merge-output-format mp4 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

or as Gabriel Staples pointed out here, the following command will typically select the actual best single file video quality resolution instead of video quality bit-rate:

youtube-dl -f best 'http://www.youtube.com/watch?v=P9pzm5b6FFY'

These commands will ensure you download the highest quality mp4 video and m4a audio from the video as a single file or will merge them back into a single mp4 (using ffmpeg in my case). If ffmpeg or avconv is not available, youtube-dl should fall back to the single file -f best option instead of the default.

Click here for more detailed information and some different examples.

Also, click to see this related answer by Gabriel Staples.

Source: www.webupd8.org/2014/02/video-downloader-youtube-dl-gets.html

Source: github.com/rg3/youtube-dl