Piracy is a war-like act committed by non-state actors (private parties not affiliated with any government) against other parties at sea, and especially acts of robbery and/or criminal violence at sea. People who engage in these acts are called pirates.
The term can include acts committed on land, in the air, or in other major bodies of water or on a shore. It does not normally include crimes committed against persons traveling on the same vessel as the perpetrator (e.g. one passenger stealing from others on the same vessel). The term has been used to refer to raids across land borders by non-state agents.
Piracy should be distinguished from privateering, which was authorized by their national authorities and therefore a legitimate form of war-like activity by non-state actors. This form of commerce raiding was outlawed by the Peace of Westphalia (1648) for signatories to those treaties.
Historically, offenders have usually been apprehended by military personnel and tried by military tribunals.

Weltweite Piraterie

Maritime piracy, according to the United Nations Convention on the Law of the Sea (UNCLOS) of 1982, consists of any criminal acts of violence, detention, rape, or depredation committed for private ends by the crew or the passengers of a private ship or aircraft that is directed on the high seas against another ship, aircraft, or against persons or property on board a ship or aircraft. Piracy can also be committed against a ship, aircraft, persons, or property in a place outside the jurisdiction of any state, in fact piracy has been the first example of universal jurisdiction. Nevertheless today the international community is facing many problems in bringing pirates to justice
While the non-wartime 20th century tradition has been for merchant vessels not to be armed, the U.S. Government has recently changed the rules so that it is now best practice (see pdf file) for vessels to embark a team of armed private security guards.

http://en.wikipedia.org/wiki/Piracy

SEE ALSO: Foreign office travel warnings (updated daily) right hand side of page and LIVE PIRACY MAPS