To decide whether an inventor is violating (infringing) a patent, it is necessary to carefully examine the patent's "claims." (Claims are terse statements of the scope of the invention, and most patents contain more than one of them.) The elements of each claim must be compared with the elements of the accused infringer's invention (usually a device or process).
If the elements of a patent claim match the elements of the device or process (called "reading on" or "teaching" the device or process), an infringement has occurred. Even if the claims don't literally match the elements of the infringing device, it is possible that a court would find an infringement by applying what's known as the "doctrine of equivalents"; that is, the patented invention and the allegedly infringing device or process are sufficiently equivalent in what they do and how they do it to warrant a finding of infringement.
For example, Steve invents a tennis racket with a score keeper embedded in the racket handle's end. The invention is claimed as a tennis racket handle that combines grasping and score-keeping functions. Steve receives a patent on this invention. Later, Megan invents and sells a tennis racket with a transparent handle that provides a more sophisticated score-keeping device than Steve's racket. Even though Megan's invention improves on Steve's invention in certain respects, it will most likely be held to be an infringement of Steve's invention, for one of two reasons:
In 2002, the Supreme Court handed down a ruling ( Festo Corp. v. Shoketsu Kinzoku Kabushiki Co. Ltd., 535 U.S. 722 (2002)) that makes it harder for patent owners who amended their patent claims to assert that others are infringing their patent. In essence, patent owners can use the doctrine of equivalents only if they can show that their amendment did not "surrender" the equivalents at issue -- a complex and confusing standard that requires the advice of a patent attorney.
In 2007, the Supreme Court modified the standard of nonobviousness applied in patent infringement cases for combination inventions and determined that "ordinary innovations" are not patentable (KSR v. Teleflex, 550 U.S. __ 2007).
If a case is brought to enforce a patent on a method or process invention, the defendant can escape liability if they were using the invention more than one year prior to the patent application date.