Voltaire once quipped that the Holy Roman Empire was neither holy, nor Roman, nor an empire. Something similar could be said about the so-called provisional patent application (PPA). Really, it's not an application for anything, it's not examined, and it can't in itself result in any kind of patent. The term is also misleading to some people who believe it refers to a "provisional patent," which doesn't exist (though all U.S. patents are temporary, expiring after 14 to 20 years). And yet, filing a provisional patent application can greatly benefit you as you start the process of protecting your invention.
A PPA is a document describing your invention, but it's much simpler and less formal than a regular patent application, and carries a smaller filing fee. It's not required to file a PPA -- you can skip ahead to the formal application if you wish. But the purpose of the PPA is to allow inventors to establish an early priority date for their idea. However, in order to hold on to that early priority date, you must file the formal patent application within one year of filing the PPA.
In most countries, it doesn't matter who first invented an idea -- the patent is given to the first person who files the patent application (first-to-file). But the United States is different in that it uses the first-to-invent system, giving priority to the true inventor. As long as the inventor was diligent in pursuing the invention (such as making a prototype), he or she will be entitled to the U.S. patent. However, if the inventor is not the first to file, this could result in costly, time-consuming interference proceedings to settle the dispute.
The United States Patent and Trademark Office (USPTO) therefore introduced the PPA in 1995 to give inventors an easier way to establish a priority date, as well as to help them in getting foreign patents. Unlike a formal patent application, a PPA is not examined on its merits. And if you fail to file a formal application within 12 months of the PPA filing date, you lose your priority date -- and nothing happens.
So, what should go into a provisional patent application?