Stack

Stacking can refer to a number of situations.

Inventory Stacking
The most obvious use of the term stacking is stacking items in your inventory. When you have a large number of items of the same type in your inventory they will generally all sit in the same slot with a number indicating how many of that item you possess. The maximum number of items that you can have in a stack is 250. Some items, such as Salvage Items, are not stackable. If you have two salvage items in your inventory then they will not sit in the same slot.

Effects Stacking
Stacking can also mean the effects of spell adding up together. Example casting one spell to give you 3 health regeneration levels and then casting another spell to give you 2 additional health regeneration levels so they stack to 5. Some effects like slowing effects and bonuses from runes do not stack in Guild Wars, only the most powerful effect is taken in to account. Also none of the effects in Guild Wars stack if you use the same spell twice.