There are many skills that affect your defense capabilities. Think of it in a common sense manner. If the skill speaks of any kind of speed (reflexes, alacrity, agility, etc) or any kind of ability to absorb damage (toughness, etc) then it improves your defense.
Obviously, each combat form has it's own level of defensive capability. You will notice that the forms go as follows (low to high) when it comes to defense:
Relaxed < Flurry < Katahzirui < Resheph
See Also: forms
For reference, these are the messages that will appear when your guild defenses are working:
Your combat intensity allows you to absorb some of the damage.
You duck, and slam your opponent to the ground with appalling force! You step to the side, grab opponents's arm while smashing your knee into his stomach! You successfully anticipate opponents's attack and strike with perfect timing. You throw opponent off-balance with your leg, grab his arm, and perform a stunning throw. You move with opponent's blow, using the momentum to perform a solid armstrike to his face!
You spin to the side with enhanced reflexes trying to avoid opponent's attack! You jump back with insane speed to avoid the attack!