You may notice just under the lower right coner of the video window, some videos have an option to “Watch In High Quality”. By default, these videos will play as a 320×240 FLV file at 320Kb/s. Check out this sample:

http://www.youtube.com/watch?v=zyyjgYcS8Fs

The user could click on this link to “Watch in High Quality” which will reload the video and play the MP4 version , which is 480×360 at 512Kb/s. By adding &fmt=18 to the end of the URL, you’ll make the URL you share with others bring them right to the high quality version. Here’s the same video in the High Quality view:

http://www.youtube.com/watch?v=zyyjgYcS8Fs&fmt=18

What else are you gaining besides increased video resolution? The video is compressed with H.264 compression, so you get better video even at the same data rate. You also go from a low fidelity mono audio feed to stereo @ 44.1Khz sample rate.

&fmt=6 increases the resolution from 320×240 to 448×336, Flash 7 video @ 900Kbps; audio @ 44.1KHz 96Kbps Mono CBR.

&fmt=18 increases the resolution to 480×360, H.264 video @ 512Kbps; audio @ 44.1KHz 128Kbps Stereo. Note, the bandwidth may be lower, but it’s utilizing a more efficient compression codec.

&fmt=22 increaes the resolution to 720p HD video if the source was uploaded at a high enough resolution.