High definition television is the broadcasting of television in a high-resolution format. At its lowest quality it is distributed at 720p, which is 720 horizontal scan lines at a 16:9 (1280x720) aspect ratio. All high definition has the aspect ratio of 16:9, although it can be cropped to 4:3. HDTV’s highest current resolution is 1080p 1920x1080 (1,080 progressive scan lines).

History of high definition
The overall idea for the creation of high definition television emerged when certain moviegoers reported feeling a more personal experience while sitting on the front few rows at the movie theater. [1] In the late 70’s Sony and NHK created a high definition TV system (NHK Hi-Vision). They introduced it to producers in the film industry in the early 80s. The NHK Hi-Vision could emulate the same clarity as 35mm film. Although the basic workings of HDTV were around in the 80s, they were not accessible to the public. The first high definition service was made available by PBS, when they broadcasted a documentary on the glass artist Dale Chihuly. By 1998 there were seven public TV stations on air. [1]

The digital transition
The United States Congress mandated that by June 12, 2009, all analog broadcasting must come to a halt, forcing the public to purchase analog to digital converter boxes in order to watch television. This move by congress, made to enable interactivity and multicasting, opens the spectrum for a much larger amount of high definition programming. [3]

High Definition formats
720p – Resolution 1280x720 progressive scan video
1080i - Resolution 1920x1080 interlaced video
1080p – Resolution 1920x1080 progressive scan video

1080i vs. 720p
Much controversy exists over whether 1080i is better than 720p. Because the 1920x1080 resolution of 1080i is interlaced, its quality is not quite as crisp as 720p. According to an article by Kenny Hemphill at Ezine, interlaced video has been altered for use with a lower amount of bandwidth [2]. While progressive scan displays the source image exactly, interlaced video displays only every other line of footage and squeezes it together playing it at 30fps. This usually causes slight pixilation and a blur when more motion occurs on the screen. 720p displays the exact image, line for line, at 60fps, thus making the clarity better. When viewing sports in high definition, 720p displays a better picture. In essence 1080i is basically 720p, but with slightly lower motion quality.

[1] http://www.hidefster.com/history/
[2] http://ezinearticles.com/?Eye-Candy?-Which-Wins?-1080i-vs-720p--And-What-About-1080p?&id=123148
[3] http://www.dtv.gov/
http://en.wikipedia.org/wiki/High-definition_television
http://www.howstuffworks.com/hdtv.htm