Priority to your invention is established by filing a US provisional or a U.S. non-provisionl patent application. Priority can also be established by filing an international patent application under the Patent Cooperation Treaty (PCT).
Provisional patent applications were introduced by the US Patent and Trademark Office (PTO) in 1995. They offer a lower cost method of obtaining "patent pending" status while evaluating or developing your idea, preparing to manufacture and market a product including your idea, or searching for a company to license your idea.
Filing a provisional application can also add up to a year to the life of your patent protection compared to starting with a non-provisional application or PCT application.
However, provisional patent applications only have a one year lifetime. To retain the priority date of the provisional patent application and maintain patent pending status a U.S. non-provisional application or a PCT application must be filed before that one year period ends.
A U.S. provisional patent application is not examined by the U.S. Patent and Trademark Office (PTO). Only when the non-provisional is filed will the PTO assign the application to an Examiner. Additional description of provisional patent applications can be found at this U.S. Patent and Trademark Office link.
Skipping the provisional stage and first-filing a non-provisional application is recommended for applicants ready to commercialize the invention and wanting less delay in issuance of the U.S. patent.
An international patent application under the Patent Cooperation Treaty affords the applicant 30 months from the priority date to file national stage patent applications in the US and/or in nearly all other countries of the world.