Defense

"Defense is your ability to avoid damage. On the other hand, damage reduction is primarily sourced from your equipment." - In-game tooltip

The Defense Skill determines your chance to fully block an attack and is required to equip most armors. Several equipment, potions, and karma skills boost the defense stat (Note: Only the "Defense stat" boosts this skill. Other Defense bonuses such as "Melee Defense" reduce damage taken and don't affect this skill.).

Experience

By default, for every point of damage dealt, the player will receive 3 xp divided between Health, Defense, and the combat skill used to deal the damage (1 xp each). This can be configured by pressing the "Edit Combat Xp Allocation" button located at the top of the skills menu to customize how the xp is divided.

Fully blocking an attack (A blue 0 hitsplat) will gain the player a small amount of Defense xp. This can be turned off in the Combat Xp Allocation menu.