When it comes to trademark (or servicemark), brand names, symbols, and logo designs are the first things to come to your mind. However, there are other types of trademarks that are protected. For example, a sound or packaging is also protected by the trademark law.
Among those, trade dress refers to packaging and other product designs that can identify a source of goods.
Below is a trade dress registered for juice drinks.
Sometimes, a part of a good itself can serve as a trade dress like the shape of guitar headstocks.
These trade dress are registered with the USPTO and afford the same protection as any registered trademark (or servicemark). As you can see, trade dresses includes only non-essential or non-typical parts of a product or packaging.
Simple and luxurious designs often require an obscure marking, and in these cases, trade dress can be a powerful protection against knockoffs.