Patents allow those who create inventions to keep others from making commercial use of the inventions without the creator's permission. Trademarks, on the other hand, are not concerned with how a new technology is used. Rather, they protect names of products and services, logos and other devices -- such as color, sound and smell -- that are used to identify the source of goods or services and distinguish them from the competition.
Generally, patent and trademark laws do not overlap. When it comes to product design, however -- say, the design of a piece of jewelry or a distinctively shaped musical instrument -- it may be possible to obtain a design patent on the ornamental aspect of the device (as distinguished from a utility patent for the invention, itself), while invoking trademark law to protect the design as a product identifier. For example, a surfboard manufacturer might receive a design patent for the appearance of its surfboard. Then, if the design is intended to be -- and actually is -- used to distinguish the particular type of surfboard in the marketplace, trademark law may kick in to protect the appearance of the board.
For more information about trademarks, see Nolo's Trademark Resource Center.
For a clear explanation of the differences between patents and trademarks, and of the entire patent process, get Nolo's Patents for Beginners, by David Pressman and Richard Stim (Nolo).