Open Game License

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.

By attaching this license game developers allow the use of their OGC and any additional content they may have indicated to be OGC. This use is perpetual, worldwide, royalty-free, and non-exclusive as long as the use is accompanied by a copy of the OGL with all the Copyrights updated to credit the contributors of all OGC that was used. All used OGC is to be indicated with a notice.

Licensees are prevented from distributing, copying, or modifying PI, and may not use the contributor's name for the purposes of marketing or advertising, unless permission is acquired through a separate license or agreement with the holders of the PI.

Finally, the OGL requires attribution be maintained by the copying of all copyright notices from OGC a licensee is copying, modifying or distributing. This requires that the license notice itself must be altered by adding all copyright notices to the Section 15 part of the license.

...
Wikipedia