High definition televisions first appeared in the late 1990s and have rapidly risen to become some of the highest profile televisions on the market. But not everyone knows precisely how they work, or why they constitute a better viewing experience than standard definition.

Digital

High definition television requires digital broadcasting, which uses less space then the older type of analog and thus can carry much more information along its wavelength.

Pixels

Television creates its imagery by using individual lines of pixels--little colored dots--which combine to make the image on the screen. While standard definition TVs have only 420 lines of pixels, HD TVs have up to 1080 lines--with a corresponding increase in image density.

Shows

Shows need to be broadcast in high definition for an HD TV to screen them in high definition. If the TV station doesn't send out an HD signal, the image only uses 480 pixel lines instead of the higher number.

Interlaced HD

Some HD televisions flicker rapidly back and forth between one half of their pixel lines and the other; the human eye can't discern the difference, though the screen tends to flicker. This type of HD television is known as interlaced, noted by an "i" after the listed pixel number (for instance, "1080i.")

Progressive HD

Unlike interlaced HD, progressive HDTV uses all of the pixel lines at once, resulting in a slightly clearer picture. This is noted by a "p" after the listed pixel number (for instance, "1080p").

Source:

Home Tech Answers: How HDTV: High Definition TV Works

National Cable & Telecommunications Commission: HDTV Technology Background