The Open Game License (OGL) is a public copyright license that may be used by tabletop role-playing game developers to grant permission to modify, copy, and redistribute some of the content designed for their games, notably game mechanics. However, they must share-alike copies and derivative works.
The OGL describes two forms of content:
Open Game Content (or OGC)
...the game mechanic and includes the methods, procedures, processes and routines to the extent such content does not embody the Product Identity and is an enhancement over the prior art and any additional content clearly identified as Open Game Content by the Contributor, and means any work covered by this License, including translations and derivative works under copyright law, but specifically excludes Product Identity....
Product Identity (or PI)
...product and product line names, logos and identifying marks including trade dress; artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs; and any other trademark or registered trademark...
Use of another company's Product Identity is considered breach of the licensing agreement.
The OGL was originally published by Wizards of the Coast in 2000 to license portions of the third edition of Dungeons & Dragons as a System Reference Document (SRD) to allow third-party publishers to produce compatible material, a move spearheaded by Ryan Dancey. Publishers could use the separate d20 System Trademark License to include a logo indicating compatibility.
In June 2008, Wizards of the Coast transitioned to a new, more restrictive royalty-free license called the Game System License (GSL), which is available for third-party developers to publish products compatible with Dungeons & Dragons 4th edition. The GSL is incompatible with the previous OGL. The OGL is irrevocable, however, and remains in widespread use.
On January 12, 2016, Wizards of the Coast released the 5th edition SRD under the original OGL marking a return to the Open Gaming format.