There are two essential aspects to "high-definition television." First, HDTV programs are broadcast in widescreen images. Many HD TV sets have wide screens—also known as "16:9 screens," meaning the ratio of the screen's width to its height is 16:9. Conventional square-box televisions have a ratio of 4:3. (Among the many nice things about a widescreen TV set is that, when you watch a "letterbox" DVD, the image takes up the entire screen; there are no horizontal black bars on the top and the bottom. Then again, when you watch an old movie or a standard-definition TV program, there are vertical black bars on the two sides.)

Second, high definition means just that—far more detail and resolution. Standard TV broadcasts have 480 horizontal lines of data. HDTV broadcasts have either 1,080 lines of data or 720 lines that are scanned twice as fast. (In TV lingo, these numbers are cited as 480i, 480p, 1080i, and 720p. The "i" stands for "interlaced," meaning the TV's "gun" scans the lines up and down 30 times a second but with each scan gets only every other line—the even numbered lines going one way, the odd numbered lines going the other way. The "p" stands for "progressive," meaning that the gun scans all the lines, up and down.) More lines mean more detail, richer colors, and smoother and more coherent (less choppy) pictures of things in motion.

To tell if a television is high-definition, look at the spec sheet's numbers for "resolution." They will say something like 840 x 520 or 1024 x 768. If the smaller of those two numbers is smaller than 720, then it is not a high-definition (or even "high-definition-ready") television. One more warning: Some spec sheets disingenuously note that the TV "receives" 1080i and 720p signals but "displays" just 480p. Such TVs are called "enhanced-definition television sets," or EDTV; they will display the widescreen image of HDTV broadcasts, but they will not have the great detail of true high-definition TV. They will look better than analogue televisions because at least the 480 lines will be subjected to progressive, not interlaced, scanning.