Apple TV hasn't hit store shelves yet -- it only began shipping to customers Tuesday -- but the impending arrival of Apple's set-top box could mean that it's time for potential Apple TV owners to upgrade their old standard-definition television sets.

Apple designed Apple TV to work specifically with widescreen, enhanced-definition (ED) or high-definition (HD) TVs capable of at least 480p resolution with HDMI, DVI, or component video connections. (See our glossary for help decoding the terms used here.) Although the Apple TV will work with an EDTV, the future of television and home movies is HD. With prices dropping on HDTVs all the time, there's no good reason to invest in EDTV's stop-gap technology.

HDTVs uses many different technologies to produce images. Tube HDTVs use cathode ray tubes (CRTs), just like traditional televisions and computer monitors. Flat-screen TVs use either liquid crystal display (LCD) technology, which is made up of pixels in front of a light source, or plasma technology, which is made up of cells filled with gases between two panels of glass. Rear projections TVs come in CRT, LCD, digital light processing (DLP; uses thousands of tiny mirrors ), or liquid crystal on silicone (LCoS; uses liquid crystals instead of mirrors).

They also come in many shapes, sizes, and prices, and each type has its pros and cons. Tube HDTVs have excellent picture quality, wide viewing angles, and are relatively inexpensive, but are bulkier and have smaller screens (34 inches or less) then other types of televisions. LCDs are thin and light; they come in large sizes (up to 46 inches or so), and their prices have fallen. On the downside, they don't offer viewing angles as wide as tube TVs. Plasma TVs are also thin and light, come in large sizes (60 inches and above), and have good picture quality and wide viewing angles, but they're also the most expensive type of HDTV and don't support the highest HD resolutions. Projection TVs (CRT, DLP, and the like) have very large screens (starting at around 42 inches and going up to 65 inches), are relatively inexpensive, and offer high resolutions; they can also be bulky, and some technologies have problems with viewing angle, color, brightness, or uniformity.

In order to view HD content, a TV needs decoding hardware to convert the signal to something your television can display. Some TVs have HD tuners built in, which is useful for displaying digital content received over an antenna. But cable and satellite companies provide their own hardware for decoding their respective signals; you don't really need a integrated HD tuner unless you plan to get your HD channels using an antenna to capture the free, over-the-air broadcasts.

As far as content from your Mac goes, Apple TV is all you need to connect to an HDTV to watch TV shows or movies downloaded from iTunes. But note that Apple currently doesn't sell HD content on the iTunes Store. However, Apple TV will let you play HD broadcasts that you've captured on your Mac using compatible hardware, although you may need to re-encode the show first. And most (if not all) of the pictures in your iPhoto library are high-definition and should look great on an HDTV.

With so many HDTVs out there, how do you decide which is right for you? A good start is to first figure out your budget and the size of set you're looking for. Once you have an idea of what you are willing to spend and how big you want your viewing experience to be, you'll only be looking at a subset of the various technologies. Then decide which features are most important to you, and try to see a bunch of TVs in action at a few different stores. (We've got some recommendations below.) And if you don't have a surround sound stereo system, it might be a good time to check those out too--most HD programming is delivered with 5.1-channel sound, as are the standard DVDs you already watch.

Glossary: Video terminology
SDTV: Standard-definition television; classic 4:3 aspect ratio set at 480i resolution.

EDTV: Enhanced-definition television; sometimes widescreen, it can display higher quality than SDTV, but not as much as HDTV. Often used as a term for televisions capable of 480p resolution.

HDTV: High-definition television; widescreen and capable of displaying at least 720p.

4:3: Square aspect ratio of SD television broadcasts.

16:9: Wide aspect ratio of HD television broadcasts.

Interlaced Scan: Used on most SDTVs and some HDTVs, an interlaced signal refreshes all odd lines of each frame and then starts over from the top with even lines; can produce flicker.

Progressive Scan: This method displays the lines of each frame sequentially, making for smoother motion.

480i: Interlaced scan video mode with 480 lines of vertical resolution and 704 or 720 pixels of horizontal resolution; this is the traditional format used for SDTV broadcasts.

480p: Progressive video mode with 480 lines of vertical resolution and 704 or 720 pixels of horizontal resolution; used on standard DVDs and for EDTV.

720p: Progressive scan video mode with 720 lines of vertical resolution and 1280 pixels of horizontal resolution; used by networks including ABC and Fox.

1080i: Interlaced video mode with 1,080 lines of vertical resolution and 1920 pixels of horizontal resolution; used by networks including CBS and NBC.

1080p: Progressive video mode with 1,080 lines of vertical resolution and 1,920 pixels of horizontal resolution; currently used only on Blu-ray and HD DVD movies.

HDMI: Digital connector that carries a high-definition video signal as well as multi-channel audio signal.

Component video: High-quality analog connector uses three separate plugs for video signal.

Toslink audio: Digital connector that carries multi-channel audio signal.

[Jonathan Seff is Macworld's senior news editor.]