Have any connections on your video card that you've never used because you don't know what they do? If you have, this is the article for you. I've gathered information and pictures about every video card connection I can think of and put them all in one article. I hope you find this reference as useful as I do, and if I've left anything out, please let me know!
You'll probably be familiar with this first set of connectors. DVI, VGA, and HDMI are all used to connect high-resolution displays like an LCD, a CRT, or a high-definition television. If you've ever even thought about putting a computer together, chances are you've seen most of these connectors already.
DVI (Digital Visual Interface)
DVI stands for Digital Visual Interface, and that's pretty much exactly what it is, sorta. Unlike a standard VGA connection, DVI can output a purely digital signal straight to your display (typically an LCD). If you have a display that supports DVI, you definitely want to use it over VGA. When using a VGA output to a digital display like an LCD screen, the image starts off in your computer as a digital signal, then gets converted to an analog signal, then once it gets to your LCD it gets converted back to digital. This conversion process causes usually causes a slight blur or shadowing effect, even when using high quality VGA cables.
DVI comes in several different flavors. By far the most popular DVI port around is DVI-I (the 'I' stands for integrated) which actually outputs both a digital and an analog signal. When you stick a DVI-to-VGA converter on the back of your video card to send output to a CRT, you are tapping into this ability for DVI to carry an analog signal. DVI-I is an ideal solution for the current transition period between CRTs and LCDs since it supports both without any real limitations.
DVI also comes in DVI-A and DVI-D flavors which are pure analog and digital solutions, respectively. I've personally never seen a DVI-A connection (a purely analog digital video interface seems silly indeed), but DVI-D connectors have shown up on a few cards. If your card is a little older you might have one of these, and note that it does not support output to an analog monitor like a CRT.
Lastly, DVI-I and DVI-D both have two variations: single, and dual link. Single and dual-link DVI connections look identical, the difference is that the dual-link version makes use of the six middle pins to increase the maximum possible resolution that can be output. Single-link DVI is limited to 1920x1200, dual-link can go up to 2560x1600. Dual-link isn't really necessary unless you have a 30" high resolution display, but it certainly doesn't hurt to have one. Currently dual-link is becoming standard on most mid to high-range current generation cards.
While nearly every DVI port you will every encounter will be single or dual-link DVI-I, there are a bunch of different incarnations of the DVI specification. All of these different connectors are beyond the scope of this article, but, if you're curious, Wikipedia has a bunch more info on the subject.
VGA (Video Graphics Array)
VGA has been with us for a long time. It's a purely analog signal designed to drive CRTs. It has a maximum resolution of 2048x1536x60Hz and is quickly becoming obsolete as LCDs replace CRTs.
DVI-I ports can be directly converted to VGA ports using a converter which takes advantage of the analog portion of DVI and gives you a VGA connector. There is no real difference between a regular VGA output and a DVI-I port converted to VGA. So don't worry if you have a CRT and you want to buy a video card that only has DVI-I ports.
HDMI (High-Definition Multimedia Interface)
HDMI is a relatively new connector starting to be used in home theater equipment. It is, essentially, a DVI port with the ability to also transmit digital audio. HDMI is still a rather rare sight on a video card, but as the price of flat panel televisions gets lower, we're sure to see more of them available for people who want to drive a home entertainment system from their computer.
Also note that to be able to use an HDMI output to display HDCP protected content (Blu-ray, HD-DVD), your card must also support HDCP. Some cards support HDMI but not HDCP, so be careful.