The d20 System is the system used at the core of Dungeons & Dragons 3rd Edition and its derived games, including d20 Modern and Pathfinder. Most products that are advertised as being compatible with the d20 System are specifically meant to be compatible with D&D; products that are meant to be used with other d20-based games will almost always explicitly specify the game in question. The d20 System and associated trademarks and logos are copyrights of Wizards of the Coast.
The core rules of the d20 System have been made available as Open Game Content in the form of System Reference Documents (both the original v3.0 SRD and later the revised v3.5 SRD). This material may be referenced and reproduced under the guidelines laid out in the Open Game License.
Third-party publishers wishing to use the official d20 System logo or to directly reference Dungeons & Dragons and its core rulebooks by name must abide by the d20 System Trademark License and d20 System Trademark Guide in order to do so. As the trademark license prohibits the inclusion of (among other things) character creation rules, it cannot be used for standalone games, only for supplements for the D&D or d20 Modern games.