A shelf was an open storage piece of furniture.
Kitchens were often equipped with shelves and books were often placed in shelves. (ENT: "Observer Effect", "These Are the Voyages..."; VOY: "11:59"; TNG: "Brothers")
The reliquary in the catacombs beneath the Vulcan monastery at P'Jem was filled with shelves which had artifacts, relics, statues, and other Vulcan objects in it. (ENT: "The Andorian Incident")
Aboard the NX-class starship Enterprise NX-01, shelves could be found in the galley, crew quarters, sickbay, and in the situation room on the bridge. (ENT: "Broken Bow", "Terra Nova", "Civilization" et al.) In the quarters small shelves were placed over the beds. Most crewmembers placed books and personal items into these shelves. (ENT: "Breaking the Ice")
The infirmary aboard the Earth Cargo Service freighter ECS Fortunate was equipped with shelves which were used to store medical supplies, jars, and thermos. (ENT: "Fortunate Son")
In 1999, Shannon O'Donnel asked Henry Janeway what he had won by fighting the building of the Millennium Gate: "The right to hide behind these shelves for the rest of your life?" (VOY: "11:59")
In 2154, Trip Tucker recalled his folks putting all the kitchen appliances on the highest shelves when he was a child so that he couldn't reach them and disassemble them. (ENT: "Observer Effect")
In 2284, James T. Kirk kept oregano in the same cabinet as dill weed. When trapped in the Nexus with Jean-Luc Picard, he asked Picard to bring him the dill, which he said could be found on the second shelf to the left behind the oregano. (Star Trek Generations)