What is HDTV?
High definition television is a new digital form of television. To be really straight to the point, HDTV just looks much better then old fashioned TV. Comparing TV with HDTV is like comparing for example a cassette tape to a CD, or a VHS tape to a DVD. The higher quality is because of the sharper images that can be displayed on a special HDTV. Furthermore the TV-signal of HDTV is digitally compressed; this allows it to contain more information.
HDTV resolutions
The quality and sharpness of an image shown on a TV heavily depends on it's resolution. Standard television (NTSC) uses a resolution of 720 by 468 pixels.
Basically HDTV comes in three types of formats:
-1280 x 720p (720p)
-1920 x 1080p (1080p)
-1920 x 1080i (1080i)
P and i stand for progressive and interlaced. Progressive means each frame contains a full picture, interlaced means that every frame displays all odd lines or all even lines. Interlaced frames succeed each other so rapidly that you will not notice the missing lines. Nevertheless Progressive scanned images often look better compared to interlaced ones.
Digital Television and HDTV
Digital television is an enhanced form of analog television. It is compressed digitally in order to send and receive more detailed information via the same 6Mhz cable. Therefore you need a decoder to decompress the digital signal it receives and send it to your (HD)TV. HDTVs have a digital receiver built into the TV itself. If you have a normal TV or a so called, "HD-ready"-TV, you will need to have an external receiver to decompress the signal. Digital television does not mean it is automatically HDTV; just some forms of digital-TV are HDTV. Like a pigeon is a bird, but a bird is not necessarily a pigeon. This is as simple as it sounds, but a lot of people tend to mix things up when they get misinformed or overloaded by information.