What Is HDTV?
High-definition television (HDTV) is all about increased resolution when compared to standard definition television (SDTV), which up until recently has been the normal viewing experience. And unlike SDTV, HDTV is also transmitted in a digital format, which means that more signals and more features can be packed into a single communications channel.
Conventional television has less horizontal and vertical resolution than HDTV, which can display programs of up to 1080 lines of resolution compared to 400-700 lines of resolution in SDTV (the exact resolution depends on which SDTV broadcasting system is being used).
The width of the image is also different in HDTV, which uses a 16:9 aspect ratio (for every 16 dots horizontally there are 9 lines vertically) instead of SDTV's 4:3 aspect ratio.
Thus when 1080 lines are being displayed, an HDTV set has a resolution of 1920 dots across. This is a very big difference when compared to NTSC, the SDTV standard used in North America, which has a horizontal resolution of 720 dots and a vertical resolution of 480 lines. The result is a much larger picture for HDTV broadcasts.
Note that the HDTV aspect ratio is close to, but not the same as, the aspect ratio used in many movies. See Why do I still see horizontal bars on my widescreen TV? for more information.