Inter Partes and Ex Parte Reexamination are proceedings with the USPTO that allow an interested party to object to the grant of a patent and point out problems. Ex parte Reexaminations can be filed anonymously by any party at any time, after which the USPTO takes over the proceeding.
If the party wants to take a more active role they can begin an Inter Partes Review, allowing them to participate in the proceeding.
Inter partes review and ex parte reexamination are powerful negotiating tools for parties worried about an intellectual property dispute, as if they are successful the patent will be narrowed or invalidated. These proceedings are an alternative to costly litigation in the courtroom and can be pursued both before litigation starts and during ongoing patent litigation.