Unless you turn off combat in the Options menu, you will occasionally have to fight enemies (Tentacles, Zombies, Ayane, etc). At the start of battle, your Constitution and Lust stats will determine your starting hp and lust. You will win a fight when you get the enemy's hp to 0. You will lose a fight when your hp reaches 0 or your lust reaches 100.
Fights are turn based. During each turn, either you or the enemy will get to perform an action.
During your turn, you can either attack, defend, or try to run away. The possible attacks depend on your Weapon. The Attack stat, which is increased by your Weapon during combat, determines the chance of hitting and damage to the enemy. Defending, despite it's name, will DEcrease your Defence until it is your turn again. How often your turn comes around is determined by your attack speed relative to that of the enemy. Armour will increase/decrease your attack speed.
During its turn, the enemy will always attack. It's attack will either decrease your hp or increase your lust. Your Defence stat, which is increased by Armour during combat, determines how much damage you take and how often you evade attacks.
During combat, the maximum values of Attack and Defence are 100 for both, regardless of what Weapon or Armour you use. Thus, doing things like wearing heavy armour when you already have 100 Defence will give you no benefits.