The J-Pac is an all in one solution for building a MAME cabinet (arcade game stuffed with a computer). It is essentially a JAMMA board that hooks right up to your computer (a JAMMA board is the most common type of game board found in arcade games, most arcade cabinets will accept one without modification). The J-PAC uses much of the same technology as the I-PAC, so please read that node for more information.
The J-PAC plugs directly into the JAMMA harness on your pre-existing arcade game cabinet (replacing your copy of Uo Poko, or whatever it is you had in there). It is recomended that you remove the power supply from your cabinet before installing your J-PAC (the power supply is not needed for the conversion, and can damage your J-PAC unit if you accidently plug it in backwards).
The J-PAC has a keyboard port which plugs into your computer, which is wired up to the existing controls in the arcade cabinet (pressing up on player one joystick sends the computer "keypad up", inserting a coin sends a number 5, etc, etc). There is a screw tag strip on the unit as well (which allows you to wire up a few extra controls, if you wish).
You plug your computers video directly into the J-PAC (use a monitor extension cord to do so). The J-PAC then routes your computers video signal to the arcade monitor. Please note that you must set your computer up in advance to send the correct signal for an arcade monitor, and this is only going to work for MAME (the arcadeOS program can reprogram most video cards to put out the proper 15 KHz signal, although Trident Blade video cards, and a few other odd cheap video cards support 15 KHz natively and can even run Windows on an arcade monitor).
The J-PAC does have a sound plug in, but it does not output sound to the JAMMA harness (basically because JAMMA standard sound is mono, and most people will want stereo. So it has a set of screw tabs for audio out (you can either wire up the original arcade speaker/s, or some of your own).
The J-PAC is the absolute fastest way to build a MAME cabinet. But it does require the use of a standard resolution arcade monitor. Some people say that those are the only monitors to play arcade games on, but I prefer a VGA open frame monitor (because of the better picture, and much wider application support).
The J-PAC was designed (and is sold), by Andrew Warne (who has a great reputation in the arcade/emulation community), and can be purchased from http://www.ultimarc.com/.