I hope to cover most questions that typically come up when folks see the terms: PGM and IGS.
Terms:
PGM = Poly Game Master
IGS = Taiwanese based arcade and PC developer, International Games Systems
PCB = Printed Circuit Board
JAMMA = Japan Amusement/Arcade Machinery Manufacturer's Association
JAMMA Edge/Connector = The universal connector used on the vast majority of Arcade PCB's over the last 20+ years.
JAMMA Harness = A female connector that receives the JAMMA Edge, with multiple wires sending the PCB's information to the hardware (arcade, supergun, etc)
SUPERGUN = An interface for JAMMA boards to display / play on a home television or other monitor. These can range from $40+ for a barebones test unit, to $500+ for a factory made unit w/all the bells and whistles and top of the line compatible arcade sticks.
CONSOLIZED = To turn a motherboard into an actual home system. This is most common with the Neo Geo MVS, though it is also done to the Atomiswave / PGM / CPS2 / F3 and other hardware.
To note, this thread will focus on the CARTRIDGE based library, and not the single board PCB's that were developed and/or produced by IGS. EDIT: Many of the PGM PCB's have been converted into cart format (early 2013).
What's so special about it? The vast majority of games are available nowhere else (save emulation). However, with emulation, getting the 2, 3 and 4 player experience is incredibly difficult, among other issues.
What type of games were produced for the hardware? Typical arcade money-makers: Puzzlers, Shooters, Run n' Gun, Fighters and Beat 'em ups.
PGM was initially designed to compete against the NEO GEO MVS and while they produced some amazing games, the arcade releases did very little outside of East Asia.
The Cartridge:
A (2) PCB cart, very similar to the MVS design. The cart connector edges are compatible only with PGM motherboard, it is not a JAMMA edge. The PGM motherboard has the JAMMA edge. Think of it as a Nintendo, you have a cart, and console (motherboard), the cart goes into the motherboard in order to be played.
The (2) PCB's:

PGM Motherboard (cart connector view)

PGM Motherboard (JAMMA edge view)

PGM Cart / Overhead w/Label View

List of Games:
(I'll include prices next to titles you should expect to pay)
Dragon World 2 - Puzzle / Tile - $20
Dragon World 3 - Puzzle / Tile - $25
Dragon World 3 Special - Puzzle / Tile - $20-30
Dragon World 2001 - Puzzle / Tile - $25-30
Demon Front - Run n' Gun - $75-95
DoDonpachi 2 : Bee Storm - Shmup - $80-100+
DoDonpachi Dai Ou Jou Black Label (conversion) - $200
EspGaluda (conversion) - $210
Ketsui (conversion) - $240
Killing Blade - 2D Fighter - $55-75
Knights Of Valor - Beat 'em up - $55-70
Knights Of Valor 2 - Beat 'em up - $75-85
Knights Of Valor 2 : Nine Dragons - Beat 'em up - $95+
Knights Of Valor Super Heroes - Beat 'em up - $75-85
Knights Of Valour Plus - Beat 'em up - $70-80
Martial Masters - 2D Fighter - $70-80+
Oriental Legend - Beat 'em up - $50-65
Oriental Legend Super - Beat 'em up - $85-100
Photo Y2K - Puzzle - $20-25
Photo Y2K 2 - Puzzle - $25-35
Puzzle Star - Puzzle - $85+
Puzzli 2 Super - Puzzle - $55-75
The Gladiator: The Road Of Sword - Beat 'em up - $85-100
PGM Motherboard - $60-90+
PGM2 (assumes with motherboard and other peripherals)
DDP DOJ Tamashi (White Label) - $450
King of Fighters Ultimate Match 98 - $400
Knights of Valour 3 - $300
Oriental Legend 2 - $250
PGM/IGS PCB only (CAVE)
EspGaluda - $300
Ketsui - $600
Teaser time: SCREEN SHOTS
DEMON FRONT


DODONPACHI II: BEE STORM

KILLING BLADE

KNIGHTS OF VALOUR

ORIENTAL LEGEND SUPER

MARTIAL MASTERS

GLADIATOR ROAD OF THE SWORD

HOLY CRAP! HOW DO I PLAY THESE CARTRIDGES THEN?
There are a couple ways to play:
Option 1 is to play on a JAMMA arcade cabinet:
Simply plug the Motherboard, with cartridge inserted, into your cabinet and go to it. This is a great way to play, however, it is not ideal for some of the 3p and 4p supported games. (there are linkable cabinets to play 4p action, but a very very few of us have the necessary tools to make this happen).
JAMMA Candy Cabinets:

Option 2 is to play on a Supergun:
Superguns can be very cheap or very pricey, depending on what you are looking for. Like most things, the more you spend, the more you get. They are capable of 4p support and any video connection you desire (SCART/COMPONENT/VGA/S-VIDEO).
Vega Supergun (Higher End Supergun)

Option 3 is to play on a Consolized PGM System:
This is done by adding AV/AC, Controller Inputs and other wiring (wiring a button to the coin, service, etc - areas of the PCB) and turning a JAMMA motherboard into an actual home system. Cost will be of the motherboard ($90-100+) and then the rest is determined by the options you want, including "encasement", if desired.
My Consolized PGM


Most Superguns and Consolized Arcade Systems use 15-pin Neo Geo Controller inputs, though in theory you can use just about anything you'd prefer to have installed. Saturn pads are used often as well.
A few options:
Neo Geo Bean Stick, Neo Geo CD Pad, and a SNES pad-hacked

EDIT:
Recently some of the PGM-PCB only games were released on PGM1 cart format, allowing them to be played on the original intended hardware, in cart format.
This is a less pricey alternative to the PCB's. Ketsui currently sells in the neighborhood of $600+, whereas the same game on conversion PGM1 cart can be had for about $250, or less than half price.
Like all conversions, this is a controversial issue for a couple of different reasons:
-Can in theory devalue the investment of the original PCB (I don't believe this personally as other common conversions such as Cyvern (Kaneko Super Nova), Punisher (CPS1), Pro Gear and Hyper Street Fighter (CPS2), Kizuna Encounter (NG AES), and Darius Gaiden (Taito F3) have all maintained or increased in value once conversion processes were made available).
-Requires the sacrifice of another game. To acquire the original PGM PCB's and an original cart shell, it is necessary to sacrifice a different PGM1 game. When discussing sacrificing, again, the controversy is tiered. To sacrifice a CPS2 game is not frowned upon much, as in theory, you are sacrificing a board that has already suicided - so what does it matter which game you should resurrect it as? However to convert an F3, Kaneko, or PGM game - you must in essence kill a (lesser) working game. When discussing legit Neo Geo AES conversions, you actually must kill both an MVS cart (to acquire the original roms) and an AES donor cart (usually a cheap fighter).
The games currently converted require the sacrifice of:
DDP DOJ Black Label = Knights of Valour Super Heroes
Ketsui / EspGaluda = Knights of Valour 2 or Martial Masters or Demon Front or Killing Blade or Gladiator Road of the Sword or DDP II
Some pics of my personal conversions in the slick PGM mini-boxes (available from "Loris Biaggi" of the neo mini-box). For information on the conversions: www.arcademodbios.com
Please ask any questions you may have - I encourage folks to youtube some vids of the PGM library, in particuler: Demon Front, The Gladiator, Oriental Legend Super, and the Knights of Valour series.