The d20 System is a role-playing game system published in 2000 by Wizards of the Coast originally developed for the third edition of Dungeons & Dragons. The system is named after the 20-sided dice which are central to the core mechanics of many actions in the game.
Much of the d20 System was released as the System Reference Document (SRD) under the Open Game License (OGL) as Open Game Content (OGC), which allows commercial and non-commercial publishers to release modifications or supplements to the system without paying for the use of the system's associated intellectual property, which is owned by Wizards of the Coast.
The original impetus for the open licensing of the d20 System was the economics of producing roleplaying games. Game supplements suffered far more diminished sales over time than the core books required to play the game. Ryan Dancey, Dungeons and Dragons' brand manager at the time, directed the effort of licensing the new edition of Dungeons and Dragons through the 'd20 System Trademark', allowing other companies to support the d20 System under a common brand identity. This is distinct from the Open Game License, which simply allows any party to produce works composed or derivative of designated Open Game Content.
Mechanically speaking, the d20 System is a derivative of the third edition D&D game system; d20 also shares mechanical aspects with other game systems including the SPECIAL system used in the computer role-playing game Fallout, the most obvious being the "feats" gained every three character levels in most games (note that this is an extension of the d20 mechanics and, while implemented by most companies, can be dropped). The three primary designers behind the d20 System were Jonathan Tweet, Monte Cook and Skip Williams; many others contributed, most notably Richard Baker and Wizards of the Coast then-president Peter Adkison. Many give Tweet the bulk of the credit for the basic resolution mechanic, citing similarities to the system behind his game Ars Magica. Tweet, however, stated "The other designers already had a core mechanic similar to the current one when I joined the design team".2
To resolve an action in the d20 System, a player rolls a 20-sided die and adds modifiers based on the natural aptitude of the character (defined by six abilities, Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma) and how skilled the character is in various fields (such as in combat), as well as other, situational modifiers.3 If the result is greater than or equal to a target number (called a Difficulty Class or DC) then the action succeeds. This is called the Core Mechanic, as opposed to the mechanics used by earlier editions of D&D, such as the first-edition hit tables or the second-edition AD&D "THAC0" and saving throw mechanics. This system is also consistently used for all action resolution in the d20 System: in prior games in the D&D family, the rules for different actions varied considerably in which dice were used and even whether high numbers or low numbers were preferable.
The d20 System is not presented as a universal system in any of its publications or free distributions, unlike games like GURPS. Rather, the core system has been presented in a variety of formats that have been adapted by various publishers (both Wizards of the Coast and third-party) to specific settings and genres, much like the Basic Role-Playing system common to early games by veteran RPG publisher Chaosium.
The rules for the d20 System are defined in the SRD (currently version 3.5), which may be copied freely or even sold.4 Designed for fantasy-genre games in (usually) a pseudo-medieval setting, the SRD is drawn from the Dungeons & Dragons books Player's Handbook v3.5, Expanded Psionics Handbook, Dungeon Master's Guide v3.5, Monster Manual v3.5, Deities and Demigods v3.0, and Epic Level Handbook. Information from these books not in the SRD include detailed descriptions, flavor-text, and material Wizards of the Coast considers Product Identity (such as references to the Greyhawk campaign setting and information on mind flayers).
d20 Modern has its own SRD, called the Modern System Reference Document (MSRD). The MSRD includes material from the d20 Modern roleplaying game, Urban Arcana Campaign Setting, the d20 Menace Manual, and d20 Future; this can cover a wide variety of genres, but is intended for a modern-day, or in the case of the last of these a futuristic, setting.
Excerpt from an article in Wikipedia.