QUESTIONS REGARDING ATTACK, DEFENSE, AND DAMAGE (RMVXACE)

I'm working on a game, and I have a question about the whole relationship between attack and defense in the engine- what's the real relationship between them? How do they affect one another? I understand that defense takes away from the amount of damage dealt by an attack, but I still don't fully understand the relationship.

Here's an example:
Character, who has 20 base attack, has a sword that has an attack rating of 10. Other than that, its attack element is physical, and it has an Ex-Parameter of 0%. In other words, it's the short sword in the RMVXA engine. Enemy has 10 defense. Does the 10 defense just ultimately cancel out the extra attack that the short sword gives and makes the enemy take damage based on the 20 base attack? If so, how would the actual amount of damage be calculated?

I just need to know the answers to this stuff so I can work on creating enemies for this game. Thanks in advance!

Ah, thanks a lot! That helps completely.