Defensive Statistics

Health: Increases the maximum health of a champion. When a champion's health reaches 0, they are killed. This statistic scales additively.

Effective Health:

Not actually in the game, it is useful for determining how much actual damage you can take.  The formula is: Health x (1+Armor/100).  For example, a 1000 health character with 50 armor has 1500 effective health.

Health Regeneration: Also known as Hp/5 or health per 5 seconds. This statistic determines the amount of health that a champion regenerates over a five second period. Each champion starts with a different rate of base health regeneration.  This statistic scales additively.

Armor: Reduces the damage a champion receives from physical attacks. Armor reduces the damage of incoming physical attacks by a percentage (called damage reduction), following the formula: Damage Reduction = Total Armor / (100 + Total Armor). For example, a champion with 150 points of armor would receive 60% less damage from physical attacks, or take 40% damage.

Armor increases a character's effective health vs physical damage by 1% per point.

Magic Resistance: Reduces the damage a champion receives from magical attacks. Magic resistance reduces the damage of incoming magical attacks by a percentage (called damage reduction), following the formula: Damage Reduction = Total Magic Resistance / (100 + Total Magic Resistance). For example, a champion with 150 points of magic resistance would receive 60% less damage from magical attacks, or take 40% damage.

Magic Resistance increases a character's effective health vs magical damage by 1% per point.

Additively: Each bonus point acquired directly affects the statistic.  For example, adding 50 health to a 1000 health character gives them 1050 health.