Darts is an ancient Earth game of hand-to-eye co-ordination and skill with roots in medieval England. A dart, a small version of an arrow, is thrown against a circular board that is divided into zones that award different point values. In the 20th century, darts became a popular recreational activity in British pubs and a casual sport around the world.
Iotian boss Jojo Krako had his own version of darts in which the dartboard was an image of his rival Bela Okmyx. This particular dartboard had targeting circles in Bela's face and his heart region. The text "Bela Okmyx" was also depicted. (TOS: "A Piece of the Action")
In 2371, Miles O'Brien, Chief of operations on Deep Space 9, introduced the game of darts to the station, placing a board in one of the cargo bays. This recreation started during Keiko's months-long absence while she was participating in a bio-survey on Bajor. (DS9: "Prophet Motive")
O'Brien convinced Quark to put up a dart board in his bar, arguing that no bar would be complete without one. (DS9: "Visionary") He and Doctor Julian Bashir regularly enjoyed playing the game. At one point, O'Brien achieved a 47-game winning streak while he was, as he put it, "in the zone", which was only halted by an unfortunately timed shoulder injury. (DS9: "Shakaar") When Bashir's genetically-engineered past was revealed, including his improved hand-to-eye coordination, O'Brien insisted that Bashir stand further away from the board in order to make their games more fair. (DS9: "Doctor Bashir, I Presume", "Business as Usual", "Statistical Probabilities")
When Worf was assigned to Deep Space 9 in 2372, O'Brien jokingly described darts as "like poker, but with pointed tips" and that it was "like target practice." (DS9: "The Way of the Warrior") O'Brien and Worf played darts together later that year, although O'Brien had just returned from Argratha and was still suffering from the Argrathi Authority's mind-altering punishment and could not remember how to play the game. (DS9: "Hard Time")
A pair of Nausicaans made a game of their own using the darts while in Quark's in 2372, which involved throwing the darts, point first, across the room into each other's chests. (DS9: "Bar Association")
Dart playing was used as a symbol for the friendly rivalry between O'Brien and Bashir. The writers of Deep Space Nine stated on the DS9 DVDs that darts was the DS9 equivalent of TNG's poker.