Here's how to identify every important cable for use with computer monitors.
VGA (aka D-Sub 15)
Use it for: Connecting PCs, monitors, HDTVs, and video projectors
It's similar in performance and use to: Component VGA
It adapts to: Mini-VGA, RGB Component
Add more ports by: Installing a new graphics card; connecting a splitter
The still-in-use analog classic, a VGA connector carries an RGB signal. You can often find one on PCs and HDTVs; laptops sometimes use the Mini-VGA version. Because the analog design can pick up interference, you're better off choosing a digital cable if your device supports it.
ADC (aka Apple Display Connector)
Use it for: Video and USB hub on older Macs
It's similar in performance and use to: DVI plus USB
It adapts to: DVI
Add more ports by: Upgrading your Mac's graphics card
Apple has phased out this proprietary display plug, but you might see it on an older monitor or Mac. Look for the rounded shape of the rectangular connector to help in identification. The cable carries power, USB, and video. You can adapt a DVI signal for an ADC monitor, but the converter boxes can be pricey; an ADC signal from a Mac is more easily converted to DVI for a standard display.
DVI (aka Digital Visual Interface)
Use it for: Connecting TVs and computer displays to PCs and other devices
If you have a choice, select it instead of: VGA, component video
It's similar in performance and use to: HDMI
It adapts to: HDMI, VGA, Mini-DVI, Micro-DVI
Add more ports by: Connecting a switchbox; upgrading your graphics board
DVI comes in a few versions, having evolved as needs have grown. DVI-I (integrated) supplies an analog and digital signal, which means that you can connect an old VGA monitor to it with a simple adapter. DVI-D (digital) carries only the digital signal. Both types also offer single-link and dual-link versions; single-link has fewer pins and can't support the massive resolutions of dual-link, but you can connect a single-link monitor to a dual-link port. HDCP, the copy-protection technology used for Blu-ray and other HD sources, works with the digital signal in DVI.