Educate me, then
<shrugs>
1. d20 is the engine that drives DND. DND is not d20
2. SRD is the material that WOTC made open for other companies to use in d20STL and OGL Products
3. Under the d20STL and OGL licenses, companies could use the SRD . However, products under the d20STL and OGL did not have to necessarily be compatible (as in plug and play from one d20 game to another) with DND which many people didn't understand when trying to mix some d20 and OGL products with DND.
The d20 STL allowed third parties to make d20 products and use the d20 logo. In exchange for using the logo the products could not include rules for character generation or advancement. The idea was that requiring the PHB for character generation and advancement would drive sales of the PHB.
The OGL allowed third parties to take the OGL and
a) make products for DND without the d20 logo or claims of compatibility with D&D
b) Take the d20 "engine" and create non-dnd games that included rules for character generation and advancement. The price for including rules for character generation and advancement was the inability to use the d20 logo. WOTC hoped that the familiarity of games using the d20 system would keep people in the d20 loop and, thus, close to DND.
c) keep supporting 3e should WOTC (or, if sold, another company) release a new edition or stop making DND since the OGL cannot be revoked.
Each license had other requirements, but that is the basics of it.