The d20 System is a system of game mechanics for role playing games introduced by Wizards of the Coast in conjunction with the third edition of Dungeons & Dragons. The system is so called because the mechanics rely heavily on rolling a 20-sided die.
The d20 is an open system which mean other designer may use it free of charge. It is not the first system of this kind That is, it can be used by other game designers (subject to certain restrictions) without paying for a license.
The d20 system isn't the first system attempting to be universal, countless came before, it is however the most popular. The runner-up for the most popular universal roleplaying system is certainly GURPS from Steve Jackson Games
Before there were as many proprietary game systems as game designers. The licensing scheme for the d20 system is known as the Open Gaming License or OGL. Followers of the Free software movement will recognize the OGL as similar to free software licenses. The intent of d20 and the OGL was to make role playing games more accessible to new players. The hope was that if other game designers took up the d20 system, that there would be a proliferation of games with similar rules. So far, a number of third parties have published d20 System games under the OGL.