The terms "Digital Television" (DTV) and "High Definition Television" (HDTV) are often used interchangeably, but they are NOT the same thing.
Digital Television refers to a TV signal that is transmitted digitally as opposed to an analog signal. Digital Television can be broadcast in several different formats, as seen in the table below. Of these formats, two are considered to be "high definition". The term high definition implies that the resolution of the digital television signal is very high. For example, the 1080i standard is considered high definition and has a resolution approximately twice that of analog NTSC television.
There are 18 "standard" digital television formats. Each one provides a different picture quality. Digital TV formats are named using a number followed by either the letter 'p' or 'i'. Here are some of the more common formats:
The number refers to the number of lines of vertical resolution . Generally the higher the number, the better the picture. For example A 720p image is much more detailed than a 480p image.
The letter refers to the way the TV makes the picture, either Progressive (p) or Interlaced (i).
A progressive scan means the TV draws line 1 of the image, then draws line 2, then line 3, then line 4, etc. until it reaches the bottom of the screen. Then it starts on the next image. This is what your computer monitor is doing right now.
An interlaced scan means the TV draws line 1 of the image, then draws line 3, then line 5, and every odd numbered line from there. Then it comes back and draws every even numbered line. This is the way regular TVs work.
You may have noticed that the image on your computer looks better than the image on your TV. With today's technology a progressive scan looks sharper than an interlaced scan. So a 480p image will look sharper than a 480i image.
Because a 'p' image looks sharper than an 'i' image, we can reduce the number of lines of resolution and still get a good-looking image. So a 720p image looks almost as good as a 1080i image.