I always described them as free-form Choose Your Own Adventure books...where you can do whatever you want and aren't just given two or three choices.
First, you create a character. How you do this varies depending on the rules of the game you're playing, but usually you choose what kind of character you want to play, then generate numerical stats that represent the basic abilities and skills of the character, typically divided into attributes (basic physical stats) and skills (learned abilities). Attributes would be strength, speed/agility/dexterity, constitution/toughness, intelligence, wisdom, willpower, charisma/appearance, etc. Skills would be something like fighting, lockpicking, gunfighting, negotiating, etc.
The gamemaster/referee/dungeonmaster (the term varies depending on the game, but it's the same role) creates the basic adventure or uses a published adventure. This would contain the plot, the stats of any characters the other players don't control, maps of locations involved, etc. Everyone sits around a table and the gamemaster describes what happens, and the players -- each controlling a character they have created -- tell what their characters do. If they try to do something that has a chance of failing (trying to attack someone, pick a lock, lie to someone, repair something, etc.), they make a roll using a die or dice (the type depends on the game being played. Shadowrun uses plain old 6-sided dice, while D&D uses a vast array of multisided dice including dice with 4, 8, 10, 12, or 20 sides and Vampire: The Masquerade uses only dice with ten sides) and the roll of the dice is compared to the skill or attribute appropriate to the task and the result is compared to a target number that varies based on the difficulty of the action. The results indicate how well or poorly the character has performed.
Here's a brief example of a game using D&D-type rules but set in a modern day:
GM = Gamemaster, P1 = Player 1, P2 = Player 2, P3 = Player 3
GM: You walk into the bar. Your nostrils burn at the thick cigarette and cigar smoke filling the air. You glance down at the photograph you were given and scan the bar for the woman. You can't seem to see her, but it's very dark.
P1: How many people are there?
GM: A dozen. There's a group of four in the corner, two guys and a girl at the pool table, a blonde at the jukebox, two men at the bar, a bartender, and a waitress.
P2: Can we tell if any of the girls are her?
GM: Make a perception test to see if you can tell.
*The GM sets a target number of 15 based on the smoke and the dim light. Each player rolls a 20-sided die and adds their modifiers for a perception test. Player 1 rolls a 12, but he gets +4 to the roll which makes the result 16 and thus meeting or exceeding the target number. Everyone else also succeeds at the roll*
GM: You look closely around the bar and realize that none of the women here is the woman you're looking for.
P2: I'm going to go to the bartender and ask him.
GM <in a deep, gruff voice playing the bartender> What can I get you?
P2: Just a coke.
GM: He pulls out a glass that still has spots on it and starts filling it from the gun dispenser.
P2: I slide a fifty on the counter along with the picture of the girl. Have you ever seen this girl?
GM: He takes the fifty, makes change, then sets it on the counter. <adopting the bartender voice> I don't know no one, understand? I ain't about to tell nothing on anyone that comes in here and we don't like people askin' questions round here.
P3: I let my jacket fall open and show him the gun I'm wearing.
GM: Make a perception roll.
*This time, the GM has set a target number of 10 (very easy) to notice the bartender's hand going under the counter, but also a target number of 20 to notice the men at the bar and pool table taking notice and looking over. The players all roll modified rolls over 10 but under 20 except for P1, who rolls a modified number of 23. The GM passes him a note saying that he has noticed the men taking notice so that the others won't know.*
P1: Oh &#^$.
GM: Are you saying that or does your character?
P1: Both.
P2: We're in trouble, aren't we?
GM: You all notice that the bartender's hand has gone under the bar slowly.
P3: I draw my gun.
GM: Everyone roll initiative.
All players: *groan*
*Initiative is a roll, usually based on speed or agility but sometimes including intelligence in order to determine the order everyone takes their actions in. P1 rolls highest, followed by Pool Guy 1, then Bar Guy 1, then P3, then Bartender, then Pool Guy 2, then P2, and finally Bar Guy 2.*
GM: Okay, what do you do?
P1: I charge at the guy with the pool cue and tackle him.
GM: Which one?
P1: The closest one.
GM: Roll your attack.
*P1 rolls his 20-sided die, adding his modifiers for attacking in order to try to roll higher than the opponent's defense score, which is based on any armor or protection he might be wearing and his ability to dodge attacks. He rolls a 13, plus a modifier of +3 to attack, gets him a roll of 16. The GM doesn't tell him the defense score of his opponent though, only if he hit or not. The man's defense is 14, so the attack succeeds.
GM: You hit, roll your damage.
*P1 then makes a damage roll based on the attack used to determine how much he has hurt or injured the man he is attacking. Since the damage roll was high and the circumstances warrant it, the GM rolls to see if the man drops his pool cue, using the stats on his sheet. The GM rolls his dice behind a three-fold cardboard screen so that the players can't see the roll. The roll fails, and the thug drops his pool cue.*
GM: You fly into the man, your shoulder driving into his stomach. He falls against the pool table and you hear a loud crunching noise as his pool cue clatters to the ground. Next, the man you just hit. He punches you *the GM rolls an attack roll behind a screen based on the player's defense score of 17 due to his long leather duster that has plates of armor in it. He rolls a 9, which with an additional +3 to attack still isn't enough to score a hit* The blow glances off the edge of your coat, and you can barely feel it. Next up is the bar guy. He picks up his stool and swings it at you *indicating player 2. He rolls, and it succeeds.* Ouch. He hits you across your back and does *rolls damage* three points of damage. *the player marks off three points of damage on his sheet, which is out of the 8 points he can take before he's incapacitated or dying* It's your turn.
P3: Did I already draw my gun?
*Players are typically limited in what they can do in each round of combat, usually one movement-type action and one attack-type action. Drawing a weapon is usually a movement-type action. You can usually, depending on the game system, sacrifice one of the two to get an extra of the other, IE not move but get two attacks or not attack, but move twice as far or draw your weapon while moving*
GM: Yes, you got it out before the crap hit the fan.
P3: I take a few steps back and shoot at the bartender.
GM: Roll your attack.
*The player makes his attack roll of 13 plus a modifier of +4, giving him a total of 17. The bartender's defense is only 14, but the GM determines that since visibility is bad due to the dim light and smoke and since his character is moving, a -4 modifier to the attack is warranted, making the modified roll a 13*
GM: You barely miss, and the bullet strikes the wall behind him shattering a bottle of Johnny Walker.
P3: Red Label?
GM: Black.
P3: Crap!
GM: It's the bartender's attack. *The GM rolls for the bartender's sawed-off shotgun -- which makes it easier to hit but decreases the damage. He rolls a 19, plus 6 for his attack roll and modifier from the gun, minus 4 for the visibility and another minus 4 for firing blind from under the bar nets him a final result of 17, which is exactly P3's defense.* It's a hit, for...*he rolls damage, then winces* 10 points.
P3: Well, it was lovely people. I'll go to the store for some snacks. Anyone want anything?
P1: You're out?
P3: Only 7 hit points. I'm done.
GM: That puts you at negative 3. If you get medical attention quickly, you might still live. But you're out of this fight. I'd like some doritos.
And that's just a basic example. It'd go on like this until everyone got tired and went home or until the complete story was done. Notice that, even though his character "died", the GM did whatever he could to make sure that the character didn't die for good. He may be out of the adventure for now, but he'll be back. I hope that helps some...and don't worry about the exact rules...I was playing a little loose with them in order to try to keep from re-writing the entire rulebook here. Have fun!
The Abstruse One
Darryl Mott Jr.