Buying a digital television can be a thrilling experience (at least for me). Figuring out which type of digital television to buy can be a bit maddening, however, if you don’t know what all of the different types mean. Here I present a brief guide to digital TV capabilities.
Digital TV logos
You may see the familiar logos such as this one for HD ready 
when you are shopping for a digital TV. The logos are there to help you quickly determine what type of digital TV you are looking at. Even though you see the logo you are looking for, I suggest at least reading the text on the side of the box (if possible) to make sure you know what you are getting. Sometimes the logos are used for display purposes and don’t give correct information about the actual TV you will be purchasing.
Digital TV types
● HDTV (high definition) – If your TV is marked as having a built-in HDTV tuner then you are ready to receive over-the-air High Definition (HD) broadcasts. All of the major network stations (e.g., ABC, CBS, FOX, NBC) broadcast some their shows in HD (mostly primetime shows and sporting events). Broadcasting shows in HD will become more popular as the February 2009 deadline for all digital broadcasts nears. HDTV signals offer about twice the resolution of SDTV signals and HDTV TVs are capable of producing a clearer more detailed picture than other digital formats. Television news anchors have commented on how a HDTV broadcast will show all the imperfections in their make-up or hairstyle. Also, you can still view analog signals on an HDTV television.
● HDTV ready (high definition ready) – A television that is HDTV ready is supposedly capable of displaying HD broadcasts, they just don’t have an HDTV tuner. HDTV ready is sometimes also referred to as HDTV compatible. Careful attention must be paid to make sure an HDTV ready TV isn’t just a standard definition TV with an HDTV adapter. If that is the case, you won’t actually be watching your favorite shows in true High Definition. I’ll save you the minute details and suggest that you look for a TV capable of a display resolution of at least 720 lines, which will be noted by either, 1280×720 or 720p. Anything less than 720 will not be capable of displaying a true HD program.
● EDTV (enhanced definition) – EDTV capable TVs are a step lower than HDTV TVs in terms of the resolution they can display. EDTV quality is the same as DVD quality, so it is a step up from your standard analog cathode ray tube (CRT) display, but not as good as a true HDTV display. TVs with EDTV technology are usually cheaper than TVs with HDTV capabilities, but that is to be expected with the less detailed picture you’ll get with EDTV. I don’t recommend buying a EDTV capable TV if you want to be able to view high definition shows in the future.
● SDTV (standard definition) – SDTV capable TVs can display resolutions higher than analog CRT TVs, but not as good as EDTV capable TVs. You may not see SDTV advertised, unless you are comparing it to standard analog TVs. SDTV capable TVs are usually the least expensive in the digital television group, but also have the lowest resolution display capabilities. Those who buy TVs with SDTV resolution, do not plan to watch HD video or HD broadcasts. The primary uses for their TV are watching DVDs and over the air analog broadcasts.
CableCARD
One other note. Some TVs are able to accept credit card-size CableCARDs which are used with digital cable broadcasts. The advantage of CableCARDs is that they replace the need for a cable box to watch digital cable. If you use a CableCARD, you free up table or cabinet space by removing the external cable converter box. A current drawback, however, is that CableCARD technology doesn’t allow for the on-screen menus that most current digital cable subscribers rely on. Digital satellite hardware doesn’t use CableCARD technology.
Link
The Home Theater Network has a good guide with pictures to help you better understand some of the features of HDTV and the differences between HD and SD (Standard Definition).