In any newspaper ad for television sets, you'll see the term high-definition used with abandon, accompanied by numbers, letters and language designed to convince you that a particular item is the one you want. Let's decipher the HD marketing-speak one factor at a time.
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.
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.
Plasma vs. LCD vs. projection vs. DLP vs. CRT: These terms refer to how the TV set physically creates the image you see on the screen. Any of these technologies might also be HD, but without more information, you just don't know.
Broadcast vs. DVD: DVDs show standard-definition pictures but generally make them look better than they would on an SD broadcast. Newer DVD players use a conversion process to make an SD picture look almost as good as HD. Until early 2008, two incompatible formats competed for true HD on DVDs, but Sony's Blu-ray won the war while Toshiba's HD DVD lost. Blu-ray players are more expensive than standard DVD players, and standard DVD players can't play Blu-ray discs.