Standard and high-quality videos
A standard-quality YouTube video has a picture 320 pixels wide by 240 pixels high and uses the Sorenson Spark H.263 video codec. The bit rate of the video signal is around 314 kbit/s with a frame rate dependent on the uploaded video.[41]
In March 2008, YouTube launched a feature which allowed some of its videos to be viewed in 'High Quality' format. This format offers the possibility of better video definition (480x360 pixels instead of the standard 320x240 pixels) for any video uploaded after this date. YouTube decides which videos are capable of this improved quality based on the standard of the original upload. Users can choose "always show me higher quality when available" on their video quality settings page in their account pages to switch automatically to the better quality.
YouTube's high-quality videos are available in two versions, both of which have a picture size of 480 x 360 pixels. By adding &fmt=6 to the web address of a video, it is played using the H.263 codec with mono sound, and by adding &fmt=18, it is played using the H.264/MPEG-4 AVC codec with stereo AAC sound.[42]
When asked why YouTube did not choose HD format, the site answered : "Our general philosophy is to make sure that as many people as possible can access YouTube and that videos start quickly and play smoothly. That's one reason why you don't see us racing to call this "Super Duper YouTube HD," because most people don't want to wait a long time for videos to play."
source:Wikipedia
