Discussion:Is D20 the same as Dnd?
From D&D Wiki
 Is D20 the same rules as DND?
I want to know if D20 and DND use the same rules.
Not exactly. 90% of the things are similar; if you understand D&D you will understand d20 Modern. However, there are some differences:
- Class structure -- modern has only 6 "basic classes", which are generic and based on each of the six stats. Advanced classes are more specialized, and prestige classes are for the absolute best in a specific area. Basic classes are modular, allowing you to pick which features you want. All types of classes include bonus feats, so you will have a lot more feats than you are used to
- Defense instead of Armor Class; every class has a defense progression that adds to your defense
- Reputation bonus -- every class has a progression for this as well
- Action points -- an option rule in D&D, action points are an important part of Modern
- Money -- instead of discrete prices, prices of items are a DC and you have to roll a Wealth check to see if you can afford it
- Cover -- d20 Modern uses degrees of cover and concealment, like D&D 3.0
- Feats -- obviously you need different sorts of feats; rules are explained for autofiring, burst firing, driving, flying planes, etc
- Spell power -- even in the highest magic campaign, you don't get to cast spells until at least 4th level
- Starting occupation -- your "pre-hero life" provides skills, feats and possibly money and fame
- Alignment is replaced with allegiance; it's not mandatory like alignment, and includes broader categories like your country or religion
- Lots and lots of little things; not game breakers by any means, just differences necessary to compensate for the fact that you are describing 21st century (or beyond if using d20 Future; 17-20th century if using d20 Past) instead of medieval times.
Edit: I'm assuming you mean d20 Modern. If you are just talking about the D20 System, then that is the system upon which games like D&D and d20 Modern are built. To use an analogy, the d20 system is a white cake mix, and you add flavors to it to customize it how you want. If you want a chocolate cake mix (D&D) you add cocoa (armor class) and chocolate frosting (knowledge: arcana); if you want a strawberry cake (d20 Modern) you add strawberries (defense) and whipped cream frosting (knowledge: technology).
Edit2: I'm also assuming you mean D&D 3.x. 4e is basically not even the same game; it doesn't play nicely with D&D 3.x, nor does it correlate with d20 Modern (which if it had to have an edition, falls somewhere between the D&D equivalent of 3.0 and 3.5).
Thanks this really helps alot.