Stands for "High Definition Televsion." HDTV is a high-quality video standard developed to replace older video formats often referred to as SDTV (standard definition television). While HDTV's video quality is one of the most noticeable improvements over SDTV, HDTV includes a number of other important improvements as well.
First of all, the HDTV signal is digital. Instead of an analog signal, used by traditional NTSC broadcasts, HDTV is always digital. This eliminates analog interference caused be electrical currents and magnetic fields. Secondly, HDTV uses a different aspect ratio than SDTV. While previous broadcasts used a 4:3 ratio (4 units wide for every 3 units tall), HDTV uses a ratio of 16:9. This wider aspect ratio more closely emulates how humans see the world, making the image appear more realistic. This ratio is also better for watching widescreen movies, which are recorded in widescreen for the same reason.
True to its name, high definition television offers a much higher resolution than standard definition video. While a typical analog broadcast in the U.S. contains a maximum of 525 horizontal lines of resolution, an HDTV signal supports up to 1080. The three formats used by HDTV are 1080i (interlaced), and 720p and 1080p (progressive). HDTV's higher resolution produces images that are much finer and contain more detail and more color than previous formats. HDTV also provides a higher-quality digital audio signal than SDTV and supports up to six audio channels compared to the two channels allowed previously.
To watch HDTV, you need an HDTV-compatible television and a means of receiving an HDTV signal. HDTVs come in both 16:9 and 4:3 formats (for backwards compatibility). Some HDTVs include HDTV tuners for receiving over-the-air broadcasts, but others require the receiver to be bought separately. Fortunately, most cable and satellite TV companies offer HDTV-compatible boxes with their digital service plans.