What is HDMI?
HDMI stands for High-Definition Multimedia Interface, and that's a pretty good description of what it is. In layman's terms, HDMI is a type of digital connection that's capable of transmitting high-definition video and high-resolution audio over a single cable. To do the same thing with analog cables, you'd need to connect three component-video cables plus six analog audio cables--that's a whole lot of cable clutter.
HDMI is typically used to connect a high-definition device--such as an HD DVR--to an HDTV. To make the connection, you simply put one end of the cable into the HDTV's HDMI input slot and the other end into the device's HDMI output slot. And that's it--just one cable and you're all set for the high-definition experience. If you have an AV receiver, just put it in the middle of the signal chain. The output of the AV receiver goes to the HDTV and you connect your high-definition device(s) to the AV receiver's input.
How does it compare to other cable types?
HDMI can deliver the best image quality of any of the cable types available today. It can handle high-definition video of up to 1080p resolution at 60 frames per second, which is the most bandwidth-intensive video format currently available. The older PC-based DVI connection offers equivalent quality, but it is rarely available on HDTVs or video components these days. Component video is found on nearly all electronics that output high-def video, and its image quality is slightly lower than HDMI, but it's really difficult for most people to tell the difference. Many viewers are probably familiar with the quality associated with the various standard-definition video cables--namely S-Video, composite (the yellow video cable), and RF--and HDMI provides a potentially huge improvement over all of them. As always, however, the biggest factor in video quality is the source; a low-quality source delivered over HDMI will still look worse than a high-quality source over S-Video.
For audio, HDMI is the reigning king of quality as well. It supports the ability to carry eight channels of 24-bit audio at 192kHz--enough to handle even the highest resolution audio soundtracks such as Dolby TrueHD and DTS-HD Master Audio. The only other connection type that can deliver the same quality are multichannel analog audio cables, but you'd need to run as many as eight separate cables to get the same quality. Digital audio cables--both optical and coaxial--can deliver multichannel audio, but are limited to lower-resolution audio signals.
Where else is HDMI used?
As with USB, there's also a mini HDMI port that is more and more commonly found on high-def camcorders. It offers the same benefits as HDMI, but the smaller size of the port makes it easier to include on portable gadgets. You can connect electronics with mini HDMI ports to an HDTV using a cable that has a mini HDMI connecter at one end and standard HDMI connection on the other, like this one. You can also use a standard HDMI cable and a mini HDMI adapter.
While DVI is still the standard on PCs, there are several new desktops and graphic cards that feature HDMI ports. While HDMI looks to be pretty much uncontested for home theater electronics for years to come, it does face competition in the computer field, as we expect DisplayPort to be seen on some systems in 2008. There's also another competing connection type, UDI, but that's still further off in the future.