ADC,or the Apple Display Connector, is a digital connection proprietary to Apple Computer, and is used on the Apple Cinema Displays. This connector carries power, DVI-D signals analog video, and USB. This allows a single cable to supply everything to an Apple monitor. All G4's, cube and other current Macs have an Apple Display Connector. To see if your Macintosh has one, check out your model's specifications on AppleSpec. 
If you wish to attach a non-apple LCD monitor to a Mac, you need to see if it accepts a digital signal. While digital is native to all LCD monitors, not all monitors have digital input connectors. Some only accept analog video. Some accept both. A digital connection is preferable since it will result in a clearer and sharper image.
Apple Cinema Displays have ADC connector. All other makes of LCD monitors have variations on the industry standard DVI connector. To attach a non-Apple monitor to an ADC-equipped Mac, you will need an adapter. Check out the DVI Extractor from Dr. Bott, which currently costs $35. This converts ADC to DVI-D.
If you do not have an ADC connector, but still want to use an ADC-equipped Apple Studio Display, check out the DVIator, also from Dr. Bott. It's an expensive adapter at $150, but allows use of all the functions built-in to these monitors.
Apple also offers a DVI to ADC adapter at their store.
Cables
If your LCD monitor did not come with a DVI-D cable, or if it is not long enough for your needs, cables are available.
The maximum cable length for DVI is 5 meters (15 feet). If you need to go beyond this length, you will need to spend fairly large amounts of money for a DVI repeater.
For a list of cable suppliers, do a search at shopper.com.
Additional monitors
The graphics card in a G4 allows for both digital and analog monitors to be used simutaneously. This can give you a substantially larger desktop work area.
If you require more than one digital monitor to be attached, you can install additional digital video cards in any unused PCI card slot.
DVI Interfacing Overview
Digital connections come in many different 'flavors'. There are seven digital connectors... DVI-D, DVI-I, DVI-A, DFP, P&D, OpenLDI and ADC (discussed above).
DVI-Dis a digital ONLY connector, and is the leading connector standard for digital only connection. DVI-D is a connection standard specified by the Digital Display Working Group.
DVI-Ican support digital and analog (RGB). The connector has a few more pins, and some display and graphics cards manufacturers are offering this connector type on their products, as opposed to separate analog and digital connectors. Be careful! Some manufacturers are not supporting analog connections, even though they are using a DVI-I receptacle! DVI-I is a connection standard specified by the Digital Display Working Group.
DVI-Ais available as a plug (male) connector only, and mates with a DVI-I receptacle. DVI-A mates to the analog-only pins of the DVI-I connector. This connector is used in adapters, where there is the need to convert to or from a traditional analog VGA signal.
DFP, or MDR-20, was the initial connector type specified by the Digital Flat Panel working group, part of the Video Electronics Standards Association. Like DVI-D, this connector supports digital only. This connector type is still used on some displays, however it is being phased out.
P&D, EVC, or M1, is another connector type that some manufacturers, such as InFocus, are using on their projectors. This connector looks a lot like the DVI-I connector, but is slightly larger in size. Like DVI-I, it also handles analog and digital. Unlike the other standards, it has the flexibility to also handle USB and FireWire video connections. Check with your display or graphics card manufacturer for compatible signals. P&D, as well as M1 are standards of the Video Electronics Standards Association.
OpenLDI, LVDS, or MDR-36, is the connector type used by Silicon Graphicson some flat panel monitors, such as the 1600SW. Although this connector looks like a DFP connector, it is wider, and uses different digital signaling. The OpenLDI connection is based on the American National Standards InstituteLVDS standard. Although LVDS signals are not directly compatible with DVI, P&D or DFP, Silicon Graphics manufactures a MultiLink adapter that converts the signal types.