Here's how the United States Patent and Trademark Office (USPTO) defines a "patent":

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.
— US Patent and Trademark Office (http://www.uspto.gov/inventors/patents.jsp#heading-1)

This outstanding video, entitled "The Patent Process: An Overview for Jurors" was produced by the Federal government to introduce key patent concepts, terms and legal frameworks to jurors in patent cases.

As an inventor, your first step - before you disclose your invention to anyone else - is to file a provisional patent application.  The process is relatively inexpensive, and can be done by the inventor without hiring a patent attorney or patent agent.  One of the best books on how to write and file a provisional patent application yourself is this one: