For at least half a century, U.S. patent law provided a fairly clear one year grace period from public use of, or offer for sale of, a utility invention before a patent application on the invention was required to be filed. In other words, the inventor could publicly use and/or put the product for sale on the marketplace and not lose any domestic patent rights, so long as the patent application was filed within 365 days of the very first time the use was public or the product was offered for sale. The consequences for violating this grace period were lost of patent rights--the invention became dedicated to the public and free for all to use.
This strict grace period was enforced after-the-fact. The inventor was required to swear in a declaration, under penalty of Federal criminal law, that the inventor would disclose all material information to the patent office--public use or offers for sale more than year before filing the application being clear material information.
If the inventor didn't tell the patent office, no one was the wiser--until litigation erupted and the inventor tried to enforce the patent in an infringement suit. Then a well funded defendant with a competent legal team would investigate every nook and cranny, interview friends and relatives, and quite possibly turn up the undisclosed public use or offer for sale. The patent would likely be held invalid, and the inventor might even have to pay the defendant's gargantuan legal fees. Thus were the strictures of the one year grace period enforced.
That was then. The Leahy-Smith America Invents Act signed into law on September 16, 2011 changes all that, as discussed in the following posts.