HD (high definition) is the highest resolution offered within the digital television category. As television programming and TV sets themselves are moving from analog to digital, it's easier to deliver movies and programming in higher resolutions. High definition content is also more often displayed in widescreen, which is similar to how it looks in the cinema. Between SD (standard definition) and HD there's a range of resolutions, but the easiest way to explain the difference is that a HD images hold more information than a standard definition images. This means that HD can be up to six times clearer than what you get on DVD, which lends itself to being shown on larger screens.

As anyone who has seen high definition content on an HDTV knows, the images are often so realistic you feel you can almost reach out and touch them. That intense realism makes the HD viewing experience, whether you're watching an action movie, a drama, or a sporting event, far more compelling and exciting than standard definition TV.

The main difference is resolution. A high definition image holds more information than a standard definition one, providing up to six times more detail. The higher the resolution, the better an image looks, particularly on a larger screen. While you may not notice a significant difference on TVs smaller than 42", on larger screens HD images retain startlingly crisp clarity in a way that standard definition simply cannot.

There are other reasons why high def makes for better viewing. For one thing, high definition content is more often displayed in widescreen format, which is closer to the way movies appear in the cinema than the traditional television format. HD DVDs also support progressive scanning, which offers even higher picture quality. They also offer multi-channel high resolution audio which is significantly better than audio on DVD.

DVD players show movies and other content at standard definition resolutions, while hi-def DVD players show content at much higher resolutions. At a mechanical level, all DVD players use red lasers to read the information on the disc, while all high definition players use blue lasers. The blue lasers can read more information on each disc, and this extra information means the movie can be shown in higher definition. High definition DVD players use special high definition DVDs that won't play on regular DVD players (since the red lasers aren't sharp enough to read them), but all of your old DVDs will play back on the new high definition players.

More information fed into your home theater speakers means deeper, richer sound. The benefits of high definition sound cannot truly be appreciated without a surround sound speaker system. We've all heard of "surround sound", but the new high definition content can offer true surround sound--enough to make you jump off the sofa!

Anyone who has experienced an HDTV playing high definition content knows how amazing it looks. But standard definition content (such as cable, satellite or broadcast TV) doesn't take advantage of HDTV's full potential--it's displayed at a lower resolution of 480 lines of resolution and uses a 4:3 (non-widescreen) aspect ratio.

Before taking your HDTV back and demanding a refund, you need to understand that an HD source is necessary to match the crystal clear video and theatre-quality audio you saw in the showroom. For an additional fee, your cable or satellite provider offers HD programming from select networks including Sky and the BBC, with more being added. A new set-top box also may be required to deliver high definition content to your HDTV.

There are several to choose from. The main two are LCD and plasma. LCD and plasma both offer "true" flat panel displays--thin enough to hang on your wall. Plasma and LCDs are rapidly coming down in price, making them more accessible all the time.

The traditional CRT television also comes in high definition, although many find the large picture tubes required for a bigger screen to be too heavy and unwieldy for many living rooms, especially as the industry is moving toward flat panel displays.

In terms of quality, LCD and plasma come in a variety of high definition resolutions and each side will argue that they offer the best experience. That's a much longer discussion, but we'll just say that each one has their strengths and a few weaknesses depending on the type of environment and content you like to watch. Read the five things you need to know before buying an HDTV in our related topic, HDTV.

Ah, the numbers and letters. We're trained to think that larger numbers are always better, and they are--sometimes. We'll come back to that in a second. First, let's start with the basics. When you watch television, the picture you see is made up of many scanned lines that compose the image on the screen. Lines of resolution vary for different digital televisions--480, 720, and 1080. The more lines you have, the clearer the image. Interlaced and progressive are the two scanning techniques that are used--that's where the "i" and "p" comes in after each number. As a rule, progressive images tend to look better than interlaced, since interlacing is a survival from the old picture tube TVs, so a 1080p image will look better than a 1080i image. Most HD programming today is 1080i, which still looks great, but it’s not the highest possible resolution. DVDs are 480p and high definition DVDs are 1080p.

Now here's where it gets confusing. The image you see is determined by a combination of the resolution of the content and the resolution that your HDTV will support. So if you play a standard DVD at 480p on a 1080p HDTV, you'll see the image at 480p--it's limited by the resolution of the DVD. By the same token, if you play a 1080p high definition movie on a 1080i HDTV, you'll see that movie in 1080i--since it's limited by the resolution of the TV. So when you're looking for a new TV, you should consider the resolution of the content you plan to feed into it. 1080p HDTVs are still the most expensive, but as prices come down and as more content is produced in this high resolution, you'll want to combine 1080p content with a 1080p HDTV.

You'll need three things: an HDTV, a source of high-def content, and the right home theatre set up. We've talked about the differences in HDTVs and in high definition content, so the final piece is a home theatre audio receiver capable of delivering multiple channels to your surround sound speakers. That will ensure you're getting mind-blowing explosions, music, and screams.

Upscaling DVD players take a standard definition DVD and "upscale" it to appear to be at a higher resolution. Despite the labels that they are "1080i" or "1080p" DVD upscalers, they are not the same as watching your DVDs in true high definition, since the source content is still limited by the content on the DVD disc itself. What you will see is some increased detail and clearer colors, however. Upscaling works best on displays with fixed pixels--namely plasma or LCD TVs. If you use them with standard CRT televisions or with some projectors, the upscaling feature may not result in a better image. It's also important to note that all HD DVD and Blu-ray players are excellent DVD upscalers.

Contrast is the ratio between the white and black parts within an image. The larger the contrast ratio of an HDTV (or any TV), the greater the difference between the brightest whites and the deepest blacks that TV can display. Therefore, a contrast ratio of 100,000:1 suggests that the black levels are 100,000 times darker than the white levels. But that's where it should end for most of us--unless you watch everything in the dark, don't get too caught up in big contrast numbers. They are largely there for show. The way your eye responds to contrast is really the important factor. The presence of even tiny amounts of ambient light in a room can make an HDTV display with a very high contrast ratio look similar to one with a much lower rating.

The back of your home entertainment system can be one of the more intimidating places in your home--to many, it's just a confusing maze of wires. To watch high definition content, you'll need the right connections for your HD cable or satellite box, or for your high definition DVD player. There are several ways to connect these devices, but the best way is with an HDMI cable. HDMI is a "secure" digital cable that protects against people trying to steal high definition movies or programming. Component cables also work for delivering video to your HDTV, but they are analog and impossible to protect against piracy, so the industry is trying to phase these out over time. But for now, they'll work for most content. On the audio side, we recommend a single optical cable that delivers up to eight channels of sound to your receiver through a fiber optic cable.

To truly appreciate the difference between standard definition and high definition, the experts say you need an HDTV that's at least 42" or larger. As you move to larger TV sizes, high definition images hold their quality. It's similar to enlarging a photograph—-as long as the photo was taken at a high resolution, even as you blow it up, it still looks great. It also depends on how close or far away you plan to sit from the TV. Check out our HDTV sizing guide, below.