Patents are available for inventions, which generally means a new and useful process, machine, article of manufacture, or composition of matter, or any new and useful improvement thereof. Design patents are available for a new, original, and ornamental design for an article of manufacture.
A patent is not available for an idea unless that idea has been turned into an invention. While there is no requirement to have a working prototype, the invention must be more than a mere concept. The inventor must be able to describe the invention in sufficient detail to allow another person to make and use the invention.
For additional information on patents, go to the Patent Office website: www.uspto.gov. The most informative page is: http://www.uspto.gov/patents/process/index.jsp