A utility patent can be either be filed as a provisional patent application or as a non-provisional patent application.
A Provisional Utility Patent application establishes a filing date, but does not mature into an issued patent unless the applicant files a regular non-Provisional Utility Patent application within one year. A provisional application includes a description and drawings of the invention, but does not require formal patent claims. There is no examination of the application, and the application is held in confidence at the USPTO.
Most importantly, since the implementation of the “first to file” rules (and abandoning the historically known “first to invent” rules) on March 15, 2013, it is important to file a patent application as soon as possible. Filing a provisional patent application that adequately describes the invention will establish priority and satisfies the need to act swiftly under “first to file” rules.
The advantage of a provisional application is usually when the applicant is in the process of developing the idea, and knows that the final implementation might change or deviate due to an improvement (or for any other reason). In such a case, the change or deviation can be filed as another provisional application, or can be directly be disclosed into the non-provisional application, when filed.
A non-Provisional Patent application is a complete patent application that is examined by the patent office. A non-Provisional Utility Patent application can claim priority (or piggy back) from a filing date of a provisional application (and thus can preserve your invention filing date). The non-provisional application requires a detailed description, formal drawings, an inventor declaration and any other information disclosure statements. It is important that the non-provisional application is crafted well as this document can eventually yield into a patent.
Another important aspect to note is that once a non-provisional application is filed, no new matter can be added into the application (note how this is different from a provisional patent application). Thus, any improvement to the technology will have to be filed into another patent application (e.g., a continuation of the non-provisional application).
However, once a prototype of the invention has been finalized, it may make economical sense to file a non-provisional application directly (skipping the provisional application) as the applicant can save the costs of preparing a provisional application.
Furthermore, in most circumstances, after 18 months of filing the non-provisional application, the application is published by the patent office and the contents of the application are made public.