Stack

Stacking can refer to a number of situations.

Inventory Stacking
Many items, such as crafting materials, will stack with other items of the same type in the one slot with a number to indicate the count. The maximum number of items that a stack can contain is 250.

Some items do not stack, such as all weapons, most Quest Items, and many Salvage Items. Two such items will not stack in the same slot even if they have identical statistics. Collectable Drops always stack.

Stacks can be split. Hold down the Shift key while dragging a stack and a dialog will pop up. The number entered into this dialog will be the number of items taken out of the original stack and placed in the destination stack.

Effects Stacking
Stacking refer to effects of spells adding up together. For example, a player could cast one spell to give +3 pips of health regeneration and then cast another spell to give an additional +2 health regeneration levels so they stack to +5. Some effects, like slowing effects and bonuses from runes, do not stack, and only the most powerful effect is taken in to account. Conditions of the same type do not stack. Benefits from spells that target enemies but also benefit the caster can be stacked; a Necromancer casting Life Siphon on two foes will receive twice the health regeneration. If you are confused, the stacking on your headpeice means that you can add a rune to the head peice to increase the effectiveness.