Dungeons and Dragons

Game Summary
Dungeons & Dragons, abbreviated D&D or DnD, is a fantasy tabletop role playing game. It was inspired by miniature war games with the rule set from Chainmail serving as the base for the initial system. D&D is widely regarded as the start of the modern role-playing game industry and servers as cornerstone tabletop RPG through its various editions.

"The first  Dungeons & Dragons  game was played back when Gary Gygax and Dave Arneson chose to personalize the massive battles of their fantasy wargames with the exploits of individual heroes. This inspiration became the first fantasy roleplaying game, in which players are characters in an ongoing fantasy story. This new kind of game has become immensely popular over the years, and D&D has grown to include many new ways to vividly experience worlds of heroic fantasy.

The core of D&D is storytelling. You and your friends tell a story together, guiding your heroes through quests for treasure, battles with deadly foes, daring rescues, courtly intrigue, and much more. You can also explore the world of Dungeons & Dragons through any of the novels written by its fantasy authors, as well as engaging board games and immersive video games. All of these stories are part of D&D."

-- dnd.wizards.com/what-is-dd

Playing the Game
The earliest edition of D&D is more like a group of rule sets collectively referred to as 1st Edition Basic and Advanced Dungeons & Dragons D&D1e as well as Original D&D or OD&D. The game play for this early system had a split between the Basic and Advanced components of the game where the mechanics varied widely. The simplicity of the basic system was meant to facilitate a rules-lite game play with more opportunity for adventuring narrative. The Advanced system focused on the detail and balance of its mechanics integrating the narrative elements as deeply into the game play as possibly it would seem.

As D&D developed it became known for the Advanced D&D or AD&D rules which was widely held as the RPG whereas the Basic D&D system was taken more as board game and only saw marginal success. For these reasons 2nd Edition D&D is effectively reducible to the AD&D2e rule set.

A good set of free-to-use rules for AD&D up through 2nd edition can be found in the Old School Reference & Index Compilation or OSRIC. PDF available here: http://www.knights-n-knaves.com/osric/download.html

Evidently it's hard to find a good rule set for any version of the Basic D&D game. 3rd Edition Dungeons and Dragons or D&D3e was a major change from a mechanical standpoint. The system aims between Advanced and Basic D&D were consolidated along with a heap of supplemental rules and streamlined. A great deal of play test balancing and an aim towards development into other titles - board and video games.

The adjustments in the 3.5 Edition revision polished the system considerably. D&D3.5e is the d20 system standard since it's an official update to the first System Reference Document or SRD created by Wizards Of The Coast, originally published along with 3e. D&D used a twenty-sided die or d20 before 3e but AD&D and the original rules aren't centered around the d20 explicitly, rather they use multiple resolution mechanics and offer several optional alternative rules. Other games employ a heavy use of the d20 often for similar reasons to the D&D mechanics but not necessarily using it exactly the same way.

One reason it's so popular is the neat way each side of the die represents 5% increments adding up to 100%. Basing rolls on die types other than say, d4, d10 or d20 means giving up clean percentage breaks or needing to switch conventions, like using specialty dice or a pool, etc. There are links to complete rule sets from the D&D3.5e SRD in the Additional Resources and References section.

Provided Dungeons & Dragons 3rd Edition indeed deserves credit as the original d20 system, the D&D5e Players Handbook credits the original game creation to Gary Gygax and Dave Arneson with the help of Brian Blume, Rob Kuntz, James Ward and Don Kaye. Further development of the framework since then is credited to people like Ed Greenwood, Margaret Weis, Tracy Hickman, Jeff Grubb, Jonathan Tweet, Monte Cook, Skip Williams, Kieth Baker, Bill Slavicsek, Andy Collins and Rob Heinsoo.

Pathfinder is often pitted against D&D 3.5e and subsequently 4e and 5e as the proper d20 spiritual successor. The system was designed by Jason Bulmahn with help from James Jacobs and Erik Mona at Paizo Publishing. A major factor in the development of the Pathfinder was the focus on play-testing and field research for fixing and improving the game play. It's worthy of consideration for players who enjoy the way D&D3.5e plays.

Need more on 4th and 5th Edition D&D as well as additional info on the board games.

Additional Resources and References
1. http://dnd.wizards.com/ - The D&D official homepage.

On the wizards.com website you'll also find the Fourth and Third edition System Reference Documents (SRD) http://www.wizards.com/default.asp?x=d20/welcome which provide information about the mechanics underlying the D&D game play. It'll be clear just by looking through them briefly the 3rd Edition SRD is a lot more helpful than 4th Edition.

2. http://www.d20srd.org/ -- The Hypertext d20 SRD by Jans Carton.

"[This site] is intended to be a useful supplement to the published rulebooks."

The Navigability of this site is exceptionally accessible compared to the standard SRD files from wizards.com.

3. http://oldguygaming.com/ Old Guy Gaming by Mike Summers

"[This] is a blog about gaming. Specifically, Advanced Dungeons and Dragons, although a great deal will apply to all versions. Other pen and paper games, as well as board games, will come up from time to time and I can’t swear that on-line games won’t make an appearance occasionally as well."

4. http://dungeons.wikia.com/wiki/DnDWiki:D20_system - Dungeons@Wikia Dungeons and Dragons Wiki - d20 System Entry

This article has a bunch of useful notes on the trademark and how licensing works.

"Because Dungeons and Dragons is the most popular role-playing game in the world, many third party publishers produce products designed to be compatible with that game and its cousin, d20 Modern. Wizards of the Coast provides a separate license allowing publishers to use some of its trademarked terms and a distinctive logo to help consumers identify these products. This is known as the d20 System Trademark License.

The d20 System Trademark License (D20STL) requires publishers to exclude character creation and advancement rules, apply certain notices and adhere to an acceptable content policy. Games that only use the OGL are not bound by these restrictions, and several have included character creation and advancement rules, allowing them to be used as standalone products. D20STL products require a core book from Wizards of the Coast and must clearly state this. As the D20STL has changed, some companies have chosen to use the OGL by itself. All D20STL products must also use the OGL to make use of d20 System open content, but publishers may use the OGL without using the D20STL.

For a long time d20 System products using one or both licenses took a significant market share of the role-playing games industry. They have especially promoted the rise of electronic publishing, since small companies can tap the huge market potential of Dungeons and Dragons at no cost to themselves. d20 System product sales, as with the rest of the industry, are currently in flux."

5. http://www.rpgattitude.com/joomla/ -- RPG Attitude

"RPGAttitude was founded on October 21, 2003 and initially started developing software applications to be used with OGL 3.5e. With the release of 4e it changed the license and they started giving them away for free to the gaming community. The website has since changed focus to developing tools to enhance gamer experiences at the table while continuing to try making life easier on dungeon masters."

6. http://www.knights-n-knaves.com/osric/ -- More OSRIC information