A utility patent is valid for 20 years. A design patent is valid for 14 years. A plant patent is valid for 20 years from application date or 17 years from patent issue date, whichever is greater. Once a patent has expired it is part of the public domain and free to use. (See Patent Extensions Below)
Patents expired for non-payment can be revived with payment. Often these patents are bought by investors or lawfirms and the fees paid in arrears. Investors and lawfirms buy these patents so they can enforce patent protections in a court of law. BDTwo.com only posts patents that have expired based on the timeframe (Utility Patents 20 yrs, Design Patents 14 yrs, Plant Patents 20yrs) to help you avoid this reality.
The United States legal system allows anyone to sue anyone for anything. The court's job is to determine the validity of the complaint. If the complaint is valid, the complainant is awarded damages by the court. If a person or company is using an expired patent (one without legal protections) the complaint is baseless, and the lawsuit is dismissed or the defendant is awarded damages in a countersuit.
Inventors and Entities which own patent rights can not extend their patent protections beyond the original grant (20 yrs for Utility Patents, 14 years for Design Patents, 20 years for Plant Patents). Once the Patent expires based on age, the invention is part of the public domain and free to use.
When the USPTO makes a mistake, denies a patent they grant later, or places a patent on hold during regulatory scrutiny (i.e. FDA approval) they can extend Patent protections (usually not more than 5 years). The Generic Animal Drug and Patent Term Restoration Act (Public Law 100-670) contained provisions for patent term restoration. Patent Term Restoration programs are governed by the Code of Federal Regulations, 21 CFR Part 60.