Standard definition (SD) vs. HD: HD always looks better, but some HDTV are better than others. Moreover, the term SD is seldom used now. If it isn't labeled HD, it's SD.

Analog vs. digital: Television broadcasting uses analog signals whose frequencies vary smoothly. Analog will disappear in February 2009 and be replaced by more efficient digital television (DTV), which can fit more channels and signals into a smaller segment of broadcast spectrum. To view DTV on an older set, you'll need a special device or nonbroadcast service -- cable or satellite -- that converts digital broadcast signals to analog. Digital provides better pictures than analog, but it is not necessarily HD. However, HD is always digital.

Definition

High-definition (HD) television refers to better-than-standard-quality, sharper pictures. How much better requires more terminology to decipher.

Widescreen vs. standard: Older TV sets mimicked then-current movie screens, with a picture 1.33 times as wide as it was high. But most movies now use a wider format, often 2.35 times as wide as they are high. To present such movies on TV requires cropping or editing the picture or showing it with black bands on the top and bottom. This "letterboxing" preserves the intended format of the original movie but can make it look tiny on smaller TV sets. With HD and DTV, the industry has a new standard screen almost twice as wide as it is high (the actual ratio is 16:9). The new format means less compromise for movies, making editing or letterboxing less objectionable. In most situations, it also provides more image content. This is especially noticeable with televised sports.