A design patent (commonly referred to as simply a "design") protects the shape or configuration of a product.
The best known design is probably the original Coca-Cola contour bottle. Any design registration for this will have expired (although it is still protected as a trademark, which can be renewed indefinitely.)
You have to be the author of the creation or have the legal rights to a design, such as the employer or the person who contracted the author to create the design.
In general you may not register a design that features Coats of Arms, flags, medals, coins or notes.
A design can be:
Aesthetic - That aspect which is judged visually only, such as the shape of a container, the pattern applied on fabric, the ornamentation on handbags or the configuration of a watch strap or clasp.
The same design may also be a registered trademark and the original artwork will be protected by copyright.
Functional - Although there may also be an aesthetic element involved, this has more to do where the shape is determined to some extent by the function of the article.
For example, a gearbox as such is not a new invention, but the gears themselves may be made to a new design (gear ratios) which can be protected by a design patent.
Circuit layouts may automatically be protected by copyright law in which case there is no requirement for registration of the design.
In some countries you can register your design for 5 years and upon paying a renewal fee, extend it for another 5 years. Another country may register for 14 years without renewal fees required, whilst another country may register for 25 years.
Unlike patents, you cannot file a provisional application for a design.
You can also register your design internationally.
More then 160 countries are members of the Paris Convention. If you have filed for registration in one Convention Country, you have six months in which to file an application for a design patent in another member country whilst being protected.