LogFAQs > #973346920

First number is attack, the letter is whether it's physical or magical, second number is phys defense, third number is magical defense. All of the numbers are actually the most significant bit of a two digit hexadecimal number (no way to view the least significant bit). When two cards fight, two random numbers are chosen. The first is between 0 and the hex value of the attacking monster's attack, the second is between 0 and the defending monster's defense of the appropriate type. The higher roll wins.

There's some more complicated stuff too (like there's an X attack type or something that will attack the weaker of the two of the opponent's defenses). Your cards also get stronger as you use them.