STATS IN RMXP.

First off, ATK and STR are not the same thing. ATK is the base amount of damage dealt by a character, this is entirely dependant on what they have equipped as a weapon, or any status changes that boost their ATK (Such as Sharp in the default). STR is a character's strength, and it is used to increase the amount of damage dealt by a weapon or skill that uses STR. At the bottom of weapon(I think weapons, I might be wrong. In which case, weapon damage is entirely boosted by the character's STR) and skill panels, you will see Parameters regarding that particular action. If you set STR to 100, it means that 100% of the character's STR is boosting the damage dealt to the enemy. This is part of the RMXP battle formulas, and the damage is then reduced by an enemy's defence. In the short run, STR is a stat given to the character as they gain levels, which influences how much damage they deal when attacking. The base damage they deal is a weapon's ATK (Not exactly, there are a few multipliers in the formula but I can't think of them off the top of my head).

As for PDEF, MDEF, and DEX: PDEF and MDEF can only be given to a character by equipment. PDEF stands for Physical Defense, and MDEF for Magical Defense. They do not give bonuses to DEX and AGI, but rather they reduce the damage that the character or enemy can take from physical or magical weapons and skills. DEX is different altogether, although I often get it confused with AGI. One of the two determines the chance of a character dodging an attack (That is non-magical), and the other determines the order in which the characters and enemies fight in during battle. Yes, if a character has no armor equipped, and their weapon gives no PDEF or MDEF bonus, then they are defenseless and will take full damage from everything.

Here's a hint for how I make ATK PDEF and MDEF work to balance battles out: If you give characters high HP (Over time) and the monsters much higher HP as they get harder, than you can give the characters large attack on their weapons, and the enemies incredibly small PDEF and MDEF so that your characters deal a lot of damage. Play around with the numbers until you get something that works for you, and I hope this helped!

Unfortunately, Evergard's explanation has errors in it.

As mentioned, ATK determines the base damage and is an equipment only stat. Standard attacks are also affected by the character's STR and the enemy PDEF. The actual formula is; Average damage = ATK - * (20 + STR) / 20.

This means that there's quite a difference between how ATK and STR affects damage. For example, if you do 0 damage, raising STR will never let you start dealing damage, but raising ATK can do that.

ATK can also affect the damage skills does.

PDEF reduces damage from standard attacks and skills with a PDEF-F higher than 0. MDEF reduces damage from skills with an MDEF-F higher than 0.

DEX affects you chance to hit an enemy with standard attacks and skills with an EVA-F higher than 0. This effect is usually minor though. DEX also affects your chance to score critical hits with standard attacks.

AGI affects your chance to avoid standard attacks and skills with an EVA-F higher than 0. AGI also lower your chances of taking critical hits, determines turn order in battle and your chance of running away from enemies.