Arrr! Criminals who specialize in robbery, traditionally, at sea. Though historical piracy and its imagery have found a place in society's imagination, its more brutal modern counterpart continues to be a problem. They are often featured in science-fiction in space environments with space-vessels as well as in fantasy settings with air or sea vessels.
Arrr! Pirates are criminals who specialize in overtaking other ships at sea and robbing them of their cargo, and sometimes enslaving their crews or holding them for ransom. Historical piracy in the Americas during the period of 1500-1800 holds a special place in society's imagination, with specific costumes, props, and accents/jargons (pirate-speak). Piracy's modern more brutal counterpart continues to be a problem in the world today, particularly off of the east African coast and in the Indian Ocean.
Historical pirates weren't necessarily exactly criminals. Often they were hired (secretly or publicly) by various country's governments to harass the navies of other nations. At other times they were a response to the brutal conscription and/or disciplinary methods of the navies of the time, and after mutiny would lead via democratic methods. The imagery often associated with pirates consists of peg legs and eyepatches and hooks for hands, the skull and crossbones "Jolly Roger" flag, parrots and tricorn hat, of buried treasure, of swashbuckling with sabers and swinging from ship to ship on ropes, and the sort of naval warfare typical of the time. As adventurous as such things sound, this often ended more tragically for those accused of piracy.