This image is from utility patent 2,974,265 
This patent is for an electric clock.

A Utility patent covers the way an invention or process works,
functions or is used, and is used for an improvement of an existing
product or process. This type of patent is for a mechanical, electrical,
or software program that performs some useful purpose. There are
two other types of Utility applications, Provisional and PCT. Page down
to get additional information about these types of applications.
Turning Ideas Into
Patents
___ Utility Patents___
A Utility Patent Application is what most people call a "Patent" This type of application can
be filed after a provisional patent has been filed of can be filed instead of a Provisional
Application. A Utility patent covers the way an invention or process works, functions or is
used, and is used for an improvement of an existing product or process. As described above,
this type of patent is for a mechanical, electrical, for software program that performs some
useful purpose.
A PCT Application is the starting point if you plan to sell your invention internationally,
and want patents in foreign countries. The Patent Cooperation Treaty (PCT) enables the U.S.
Applicant to file one application, “an international application,” in a standardized format in
English in the U.S. Receiving Office (the U.S. Patent and Trademark Office), and have the
application acknowledged as a regular national filing in as many member countries to the
PCT as the applicant “designates” or “elects,” that is, names, as countries in which patent
protection is desires.
A Provisional Application basically just holds your place in priority. It is best used if you
have a working idea and want to ensure that no one files for a similar patent before you can
perfect your design. Provisional applications are basically the same as utility applications
except for three major and one minor area. The three major differences are that the application
does not include claims, the application is not examined, and the inventor has one year to
apply for a utility application or the application dies. The minor difference is that the cost to file
the application is about one fourth the cost of a utility application. Besides the cost the major
advantage of filing a provisional application establishes a date on invention.
International PCT Applications
When should you file a Utility Patent Application? Many people file for
this type of application when they have a working prototype.   If you have
an invention that is fairly well defined, and you are in the process of final
tweaking of the design, or initial stages of production. You have up to one
year to file for a Utility Application after you first showed or sold the
product. After the patent issues you will have patent rights for 20 years
from the date you filed the application with the patent office.
A Provisional application has a life of only one year. Prior to the end of the year you must
file a Utility Patent Application or the provisional application dies, and your rights of priority
die with the application. 
When should you file a provisional application? If you have an invention that is fairly well
defined, and you are in the process of tweaking the design, and would like to perform some
market testing and adjustment based upon customer feedback and you want to ensure that if
someone sees your invention you have some level of patent protection in place then a
provisional application may be a good place to start. You should contact a patent law
professional at our office to determine if a provisional application is best for you.
35 U.S.C. § 101 Inventions patentable
Whoever invents or discovers any new and useful process,
machine, manufacture, or composition of matter, or any new and
useful improvement thereof, may obtain a patent therefore, subject
to the conditions and requirements of this title. This includes finding
a new non-obvious use for an item and an improvement to an existing product. A utility patent
can cover not only the invention but also how the invention is used.