"Dishwasher safe" is a designation given to items that can be cleaned using an automatic dishwasher without any true risk of damage or wear to the item. While your dishwasher will clean most items in your kitchen without any hassle, some items should never find their way behind the locked door of your dishwasher's rinse cycle.
Most plastics are completely dishwasher safe, it's true. However, there is one thing to watch out for: when placing plastic containers, utensils and other items into the dishwasher, avoid putting them on the bottom rack, if at all possible. The reason for this is because plastics tend to vary in heat tolerance. While certain plastics will be fine on any rack in your dishwasher, others may warp or melt. Likewise, plastics that are disposable and are not marked as "dishwasher safe" may not be suited to the high temperatures of any dishwasher and shouldn't be washed in one.
Metals, such as stainless steel and aluminum, are practically designed to be dishwasher safe. Unless otherwise indicated on the product, you won't have any issues washing your stainless steel and aluminum pots, pans and cutlery. Be warned, however, that you always should rinse pots and pans before putting them into the dishwasher. Food residue often can damage the finish on metallic kitchenware.
Certain metals should not be washed, as the high exposure to water and steam may cause them to become damaged or rust. Cast iron, tin, bronze, pewter, gold and brass will rust or discolor over time in a dishwasher and should be hand-washed instead.
Glass, China and Stoneware
While the majority of glass, china and stoneware items are dishwasher safe, certain items still should be washed by hand. Hand-painted items, lead-crystal glasses and antique items may be cleaned best manually, as the dishwasher may be too harsh for the finish and cause further wear or discoloration. Generally, most of these items will have a designation that they're dishwasher safe. If you're unsure and the item holds a great value to you, don't take any chances.
Don't put wooden spoons, wooden cutting boards or utensils and kitchenware with wooden handles in your dishwasher, as this will likely cause the wood to wear, warp, crack and discolor. Instead, always take care in hand-washing these items, this way they last as long as possible.