Physical damage

Physical Damage is one of  three types of damage in League of Legends, the others being True Damage  and Magic Damage. Physical Damage is dealt by all champion autoattacks and by certain champion abilities; the number of physical Damage abilities is very little compared to magical damage abilities.

Statistics influencing physical damage
The Physical Damage of a skill or of Autoattack can mainly be boosted with Attack Damage items. Your total damage is calculated by the damage gained from boosts + your basic damage. The Damage of some Physical Abilities can be increased with AP as well.

The amount of Physical Damage received can be reduced with Armor - all Armor points are added to calculate a damage reduction percentage. The defender's Armor can be decreased by your Armor Penetration.

An indirect way to boost physical damage is to increase either the Attack Speed or the Crit Chance. This will grant either more hits per second, or a higher chance to hit a critical (200% of the normal damage), which is the base element of many melee item builds.

The critical damage can be altered by :
 * runes.
 * The mastery.
 * The.
 * The.

Calculation example
Putting everything together, you receive the general formula for Physical Damage:

PhyDmg = (BasicDmg + DmgBoost) * (1 - ((Armor-ArmorPen) / (100+Armor-ArmorPen)))

Lets say you have a base damage of 55, your damage boost is 73, which makes a total of 128. You have 20 Armor Penetration and the opponent has 55 Armor.

Using the general formula, you will hit your opponent for (55+73)*(1-((55-20)/(100+55-20)))=94.81 = 95 Physical Damage.