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 do not stack, like weapons dropped from enemies, Quest Items and many Salvage Items. If you have two such items in your inventory then they will not sit in the same slot even if they have identical statistics. Collector Items always stack.

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 on the same target. (e.g. if a warrior causes bleeding on a foe and then a ranger causes bleeding too, this will not make the foe bleed twice as fast, though it will renew the effect.) If an offensive spell is cast that has a benefit to the caster, then the benefit stacks provided that the spell is cast on multiple targets. (e.g. if a necromancer casts Life Siphon on one foe and then on a another foe, then he/she will have twice the health regeneration.)