A postcard was a souvenir in the form of a postal letter. Postcards contained images, often photographs of special landmarks or objects with room at the back of the card to write a letter.
In a holographic recreation of 1944, Brigitte had promised to write a postcard to Bobby Davis, which was prevented by the outbreak of the war. When Bobby encountered Brigitte in Sainte Claire, he told her she owed him a postcard. (VOY: "The Killing Game")
While inspecting the prototype NX-class starship Enterprise from the outside in 2151, chief engineer Tucker jokingly asked Captain Jonathan Archer whether he planned posing on Enterprise's hull for some postcards, when Archer insisted newly installed hull plating match the color of the nacelle housings. (ENT: "Broken Bow")
When the USS Rubicon prepared to enter a subspace compression anomaly in 2374, Jadzia Dax told Benjamin Sisko she'd send him a postcard, when Sisko expressed sorrow for not going along to explore the anomaly. (DS9: "One Little Ship")