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: