Patents expire after a maximum of 20 years after the filing date. (Special extensions can be granted by the government, but these are extremely rare.)
The government recognizes 5 "rights" periods in the life of an invention.
1. Invention conceived but not yet documented: When an inventor conceives of an invention, but hasn't yet made any written, signed, dated, and witnessed record of it, the inventor has no rights whatsoever.
2. Invention documented but patent application not yet filed: after making a proper, signed, dated, and witnessed documentation of an invention, the inventor has valuable rights against any inventor who later conceives of the same invention and applies for a patent. The invention may also be treated as a "[tradesecret" - that is, kept confidential. This gives the inventor the legal right to sue and recover damages against anyone who imorally learns of the invention - for instance, through industrial spying.
3. Patent Pending - patent application filed but not yet issued: During the patent pending period, including the one-year period after a provisional patent application is filed, the inventor's rights are the same as in Period 2 above. A patent application gives an inventor no rights whatsoever - only the hope of a future monopoly, which doesn't commence until a patent issues. However, most companies that manufacture a product that is the subject of a pending patent application will mark the product "patent pending" in order to warn potential copiers that if they copy the product, they may have to stop later (and scrap all their molds and tooling) if and when a patent issues. The PTO by law must keep all patent applications preserved in secrecy. The patent pending period usually lasts from one to three years.
4. In-force patent - patent issued but hasn't yet expired: After the patent issues, the patent owner can bring and maintain a lawsuit for patent infringement against anyone who makes, uses, or sells the invention without permission. The patent's in-force period lasts from the date it issues until 20 years from its filing date, provided maintainence fees are paid. Also, once the patent issues, it becomes a public record or publication that can block others who file later from getting patents on the same or similar inventions - that is, it becomes "prior art" to anyone who files after its filing date.
5. Patent Expired: - after the patent expires (20 years after the filing date, or sooner if a maintainence fee isn't paid), the patent owner has no further rights, although infringement suits can be brought up for any infringement that occurred during the patent's in-force period. An expired patent remains a valid "prior-art reference" (as of its filing date) forever.