Many items are subject to wear/rotting/withering if left outside.
There are three levels of wear, which are signified by, in order of increasing wear, x, X and XX bracketing the item. Viewing will give an account of how badly worn(with some wear...mangled) the item is. Item value is significantly affected by the level of wear.
Most items are not subject to wear when stored in stockpiles, or inside, notably prepared meals, drinks and plants. Some items, in particular plants, bags, and cooked meals, will still rot when lying around inside, outside of stockpiles, in workshops, or, worse, in water.
Certain types of vermin, if left unchecked, can cause wear on food even when stored in stockpiles. Storing food in barrels can reduce the amount of wear, and metal barrels are observed to provide better protection.
Plants will not wither on stockpiles, but will wither on fields if not harvested in time.
Mangled food can still be cooked, seemingly without quality loss, although the value of the prepared food will be calculated using the reduced value of the ingredients.
The remains of vermin, as well as corpses and chunks, will rot both inside and outside, more importantly they will cause miasma inside. It is therefore necessary to keep an eye on tidying up and either have refuse stockpiles outside or separated by doors from the rest of your fortress if inside.
Clothing will show wear over a period of years when it is being worn by a creature. Currently, no unhappy thoughts result from this wear.
All improvable items made from cloth, silk, or leather will gradually wear out over a very long period of time, even if just sitting in stockpiles, gaining one level of wear every 20 years. Items made of wood wear out even more slowly - one level every 100 years.