Differences in structure, function or operation may be the basis for seeking patent protection. A patent is a property right granted by the Government of the United States of America to an inventor "to exclude others from making, using, offering for sale, or selling the invention throughout the United States or importing the invention into the United States" for a limited time in exchange for public disclosure of the invention when the patent is granted.
Design patents cover the unique appearance of your products. Think of them as protection for your industrial designs since they do not cover the structure or function of the device. Design patent protection can be quite powerful and very valuable. This is especially true when design patents are used with other forms of intellectual property. If you have product designs that are unique, failure to file an application may result in your being barred from ever receiving a design patent on that product. If you have made and used the design in public or offered the product with that particular design for sale more than one year before the file of a patent application on that design you cannot get a patent in the United States on that design. Design protection is both an offensive and a defensive strategy. It allows you to assert ownership over all that is yours and prevent others from encroaching in on your share of the market.
The patent law provides for the granting of design patents to any person who has invented any new, original and ornamental design for an article of manufacture. A design patent protects only the appearance of the article and not structural or utilitarian features.
35 U.S.C. 171 provides for designs patents. It states:
"Whoever invents any new, original, and ornamental design for an article of manufacture may obtain a patent therefore, subject to the conditions and requirements of this title."
A design consists of the visual ornamental characteristics embodied in, or applied to, an article of manufacture. Since a design is manifested in appearance, the subject matter of a design patent application may relate to the configuration or shape of an article, to the surface ornamentation applied to an article, or to the combination of configuration and surface ornamentation. An ornamental design may be embodied in an entire article or only a portion of an article, or may be ornamentation applied to an article. A design for surface ornamentation is inseparable from the article to which it is applied and cannot exist alone. It must be a definite pattern of surface ornamentation, applied to an article of manufacture. If a design is directed to just surface ornamentation, it must be shown applied to an article in the drawings, and the article must be shown in broken lines, as it forms no part of the claimed design.
35 U.S.C. 173 sets forth the term of a design patent: It states:
"Patents for designs shall be granted for the term of fourteen years from the date of grant."
A design patent application may only have a single claim (37 CFR § 1.153). Designs that are independent and distinct must be filed in separate applications since they cannot be supported by a single claim. Designs are independent if there is no apparent relationship between two or more articles. For example, a pair of eyeglasses and a door handle are independent articles and must be claimed in separate applications. Designs are considered distinct if they have different shapes and appearances even though they are related articles. For example, two vases having different surface ornamentation creating distinct appearances must be claimed in separate applications. However, modified forms, or embodiments of a single design concept may be filed in one application. For example, vases with only minimal configuration differences may be considered a single design concept and both embodiments may be included in a single application.
In general, a "utility patent" protects the way an article is used and works (35 U.S.C. 101), while a "design patent" protects the way an article looks (35 U.S.C. 171). Both design and utility patents may be obtained on an article if invention resides both in its utility and ornamental appearance. While utility and design patents afford legally separate protection, the utility and ornamentality of an article are not easily separable. Articles of manufacture may possess both functional and ornamental characteristics.
http://www.uspto.gov/patents/resources/types/designapp.jsp#types - A guide for filing a Design Patent Application.