From D&D Wiki
Supplemental Variant Rules
A supplemental variant rule can be implemented into a campaign with little or no change in the core dynamics. These rules cover an area that the core rules do not cover or do not cover well.
Transformational Variant Rules
A transformational variant rule, when implemented into a campaign, will change the core dynamics. These rules change the core rules to make them more playable or more realistic while staying within the d20 framework.
Radical Variant Rules
These are variants which completely overhaul the mechanics of the game both large and small, and often bear only a resemblance to the d20 system.
These have yet to be categorized for how they implement into typical D&D rules. Please help and categorize these.