In this case, the Wikipedia article does a really good job of defining the game: it is “a fantasy tabletop role-playing game…” where each player controls “a specific character” and, as a group, “solve dilemmas, engage in battles, and gather treasure and knowledge.” “A Dungeon Master serves as the game’s referee and storyteller while maintaining the setting in which the adventures occur, and playing the role of the inhabitants.” The article goes into greater detail about the game’s history, mechanics and conventions, with each sub-section linking back to an internal primary article and to external sources (as is the standard for Wikipedia).
Where the article is lacking is the specifics of the game. D&D has been published under several editions, with a few of those boasting sub-editions; and it has inspired quite a few spin-offs that superficially appear different, but are really very much the same in their core mechanics. Thus, I offer my definition:
D&D is a role-playing game with specific game rules, conventions and components that separate it from other RPGs. It uses certain mechanics to describe characters and the world around them; it groups characters into classes or roles; it’s set in a fantastic world (often medieval, sometimes futuristic, but always distinctly different from our own); and uses a risk/reward system centered on fighting creatures (or non-player characters) and taking their stuff.
D&D uses a core mechanic: the d20. The dodecahedron is called on to determine the outcome of any action where failure is in question. A player (PC) decides to do something; the dungeon master (DM) recognizes the action has the potential to fail; the PC rolls a d20 and checks the result against a known value. Sometimes the PC has to roll high; sometimes low; either way, the die determines success while the PC’s abilities or skills determine the likelihood of success (by modifying the target value and/or the die roll).
This core mechanic is not necessarily used for every situation. The earliest versions of the game used a d100 for some skill checks. Many a homebrew version use different rolls, such as 2d6 for morale checks. Regardless, certain character traits always use the d20: ability scores, attack rolls and saving throws.
Ability scores are natural traits possessed by all characters. They are rated on a scale of 3 to 18. In some versions of the game, the PC has to roll equal to or under his ability score to succeed at a trial. In other versions, the ability score provides a bonus to the d20 roll and the PC compares his result to a different trait (or a value assigned by the DM).
Attack rolls also require a d20 and the goal is to roll high (either against the PC’s attack trait or the opponent’s defense trait; in either case, a low trait is good for the PC). A successful roll indicates a hit and the PC rolls damage (determined by the weapon and/or the PC’s skills), subtracting the result from the target’s hit points.
Saving throws are similar to attack rolls in that the PC wants to roll high, but differ because they are reactive instead of proactive. The PC declares his attack against an opponent; he does not declare his defense against an attack. Saving throws are used primarily for magic and to protect against monsters, but may also apply against the PC’s environment (as determined by the rules).
As noted above, variations on this core mechanic are common from edition to edition, and especially from table to table.
D&D assigns a character class to every PC. The full range of class options varies with the game’s edition, rules and DM preference; but every character has a class (and some might have two or three). A PC’s class determines his current skills and potential to acquire new skills.
The D&D classes are grouped into four general categories: warrior, priest, rogue and wizard.
Warriors are the champions, swordsmen, soldiers, brawlers, gladiators, guardsmen, militia, bandits, knights or one of many combat-oriented professions in the world. They are specialized in battle and warfare, masters of personalized hand-to-hand combat. They are physically adept and skilled in performing feats of strength or endurance. They are the military leaders in the world, capable of judging persons and situations, of making quick assessments, and of developing and executing complex strategies.
Priests are Templars, counselors, healers, preachers, curates, elders, shamans, legislators, or one of many religious or political professions. They are a deity’s representative on earth. They are trained in the secrets of the afterlife capable of guiding persons to their rightful place. They are leaders in the spiritual and political sense, capable of wielding divine energies and passing judgment on matters both eternal and mortal.
Rogue are cutpurses, bandits, pickpockets, charlatans, street rats, beggars, burglars, embezzlers or one of many criminal professions. They are specialized in illegal activities, acting in a variety of devious, underhanded ways to circumvent laws and physical obstacles. They are the scoundrels of the world, as likely to give you a purse of gold as they are to slit your throat in the night.
Wizards are alchemists, diviners, occultists, conjurers, illusionists, magi, sorcerers, scholars, philosophers or one of many arcane and scientific professions. They have studied the world, cultivating an appreciation for math, physics, chemistry and a half-dozen other disciplines. Their training grants them to ability to see connections most people miss. They wield arcane energies much as a weaver wields a loom or a musician plays an instrument ~ they appear to change reality with a thought, but their methods are not so strange to the initiated.
Many other classes, sub-classes and advanced classes might exist in a D&D game, from barbarians to monks, bards to druids, or illusionists to warlocks. Each of these is associated with one of the four categories above; the grouping determines the sort of abilities the class possesses. For example, bards in different editions have been a base class (under the thief category), a sub-class (a specialized thief) and an advanced class (where the PC must take levels of fighter, wizard and thief before becoming a bard); each version has had abilities that matched with its classification.
Magic and Monsters
D&D assumes that ~ wherever the game takes place, whatever the fantasy world ~ there will be some element of magic and monsters.
D&D magic operates according to specific rules, such that each magical effect is clearly defined and limited in scope. The specifics of a D&D magic system ~ traditional “Vancian” spells, AD&D psionics or even 4th Edition’s at-will/encounter/daily powers ~ will vary from edition to edition, from table to table. What they all have in common is that they are not easily manipulated; their rules are not easily re-interpreted; in other words, the players cannot simply decide to use a fireball spell to light a cigarette.
D&D games also feature the clear presence of creatures and monsters. The full range of creatures varies, again, from one game to the next; but will often include such notable staples as dragons, goblins, orcs, giants, elves, dwarves, and so. This is another assumption of the game, that there are fantastic creatures in the world just as there is magic available to mortals.
Risk and Reward
The core conceit to the game is that it will feature adventures: whether the DM presents adventures to the PCs or the PCs go in search of adventure, the PCs will make enemies and allies; fight battles earn reputation, experience, treasure and power; and so on. The classic adventure example is the dungeon. It has an entrance, a fairly linear path and a clear exit. This is by no means the only form that adventure can take; it’s just the most common and the easiest for new DMs to run. It’s also a good standby option for players who want a few sessions of simple kill-it-all-and-take-its-stuff.
It is during an adventure that the players are exposed to risk. The DM presents the adventure, the players assess their ability to defeat the challenges in the adventure, and they engage those challenges in a manner of their choosing. In defeating their enemies, they earn experience and treasure; in earning experience, they acquire levels and additional power or skills.
The primary risk in the game is the loss of characters. Other things may be lost, from hit points and spell slots, to equipment and possessions, to followers, reputation, land or power. These losses, while significant to the player, may be recovered in the future. The loss of a character is more permanent. (Even though magic makes character revival a possibility, each version has placed some limitation on that recovery, such that the players must choose how much additional risk they’re willing to accept in the pursuit of reviving a fallen character.)
These are not the only reward options but they are the ones that have been written into the game’s rules.