What's the Difference Between OGL and d20?


I'm confused. Being relatively new to RPGs (two years), I not sure what the difference between OGL and d20 is. I *think* OGL means that the mechanics definded by the SRD are used but that the game is stand-alone, not requiring the D&D/Modern PHB, DMG or MM. On the other hand, d20 relies on the gamer having D&D or Modern PHBs.

Thus a d20 game must be OGL, but the reverse need not be true. Can someone from Mongoose confirm, deny, or clarify please?
That is in fact the simplest way of viewing it.

D20 logo carries a certain notoriety with it. If you want you book recognized, you have to put that "This product requires use of blah blah blah published by Wizards of the Coast" line. There are certain things you can't have in your book if you use the D20 logo as well.
D20 products can't include the basic rules for generating characters and the experience points/level table. Thus requiring the D&D PHB.

OGL can. And cannot have the D20 logo.

The D20 logo presumably generates additional sales.
The OGL is not actually a rules system, it refers to the Open gaming Licence. Any material that is published under the OGL can be reused by other companies free of charge. Wizards of the Coast made most of their D20 system available under the OGL, except for the character creation stuff - presumably to ensure sales of their D&D Players Handbook.

Most people tend to use the term OGL and D20 interchangeably, however that is technically incorrect. It is possible to release any RPG game under the OGL, and it doesn't have to bear any similarities to the D20 system at all.

In fact Goldrush Games has already done this with their Action! system, which uses am Attribute + Skill + 3D6 system. Unfortunately people's use of the term OGL to refer to D20 does nothing to help their sales.

Mongoose's use of the term OGL rules is therefore really a misnomer (sorry Mongoose!). I even had to contact Mongoose's sales support to check that what they referred to as OGL rules actually meant "the ruleset of the popular game system that happens to use a D20".

Vist the Open Gaming Foundation to learn more about the OGL, the systems released under it, and the fact that there are other licences that could be used as well as the OGL.

Wizards of the Coast also have a licence that covers the D20 system and logo. This licence can be changed at their whim and any publisher that wants to use the D20 logo must abide by the rules set out by WOTC.

WOTC maintain quite a strict control over the D20 licence and even changed it specifically to prevent The Book of Erotic Fantasy from being published under the D20 logo (it was still released under the OGL however, which WOTC have less control over).

Mongoose can publish "D20-like" game systems under the OGL and make them stand alone as they can include a character creation system (the D20 licence I believe prevents any publisher doing that) as long as it isn't the one outlined in the D&D PLayers Handbook (as that one is presumably cpyrighted by WOTC). Therefore I don't think you'll find the "Roll 4D6 and drop the lowest result" method in the OGL Rule Books and Conan.