Hits against any target can potentially cause a Stun on that target, depending on the damage dealt. Stunning a target interrupts their current action and prevents them from taking actions for a short time. There are two types of Stun:

Light Stuns last a fraction of a second but can be inflicted frequently. Any Hit has a chance to cause a Light Stun. The chance is based on the damage dealt, up to 100% base chance for Hits that deal 100% of the target's maximum Life. Chances lower than 15% are treated as 0%.

Heavy Stuns occur when a target's Stun bar is filled and last multiple seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).

Player Physical damage and Melee Damage each have 50% more Light Stun chance and Heavy Stun buildup. These bonuses are multiplicative with each other.
Monster Physical damage and Melee Damage have 100% more and 33% more Light Stun chance and Heavy Stun buildup respectively. These bonuses are multiplicative with each other.

Heavy Stuns occur when a target's Stun bar is filled, interrupts the target's current action and prevent them from taking actions for a few seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).

While an enemy is Heavy Stunned, they cannot act and count as Immobilised. It will also be harder to Heavy Stun them again for a short time afterwards.

Physical damage, and player (but not monster) Melee Damage, each cause 50% more Heavy Stun buildup. For players, these bonuses are multiplicative with each other.

Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount). A player's stun bar only empties when they are not taking these actions or Heavily Stunned. While Heavily Stunned players cannot Block, Deflect or Evade.

Chance to Block only includes modifiers that are explicitly chance-based. Guaranteed Block (such as raising your Shield) does not affect your Chance to Block.

Armoured Shields are any Shields that grant the player Armour — that is, any type of Shield other than Bucklers.

Bucklers are a special type of Shield that do not grant any Armour and can Parry enemy skills instead of being raised to Block.

Equippable Armours are pieces of equipment that you can wear to provide various defences. Equippable Armours include Helmets, Body Armours, Gloves, Boots, Shields and Foci.

Depending on the type of Armour equipped, Players will have a less Movement Speed penalty applied depending on what Attribute the Armour requires. These penalties only apply to equipped Body Armours and Shields.
For Body Armour, pure Strength has a 5% penalty, hybrid Strength and Dexterity or Intelligence has a 4% penalty and pure Dexterity or Intelligence has a 3% penalty.
For Shields, pure Strength has a 3% penalty, hybrid Strength and Dexterity or Intelligence has a 1.5% penalty and pure Dexterity or Intelligence has no penalty.