The Poké Mart is a market for Pokémon Trainers. Almost all of the towns/cities in the Pokémon World have one, some of which in a department store-like format. These are essential to purchase any supplies that might be needed, such as Poké Balls, Potions and items to heal Status Conditions such as Antidote.
During the progression of the game, Poké Marts will start to sell better merchandise. During the first few Generations this depended on the town that the player is currently in. Starting in Generation IV, however, the availability of the merchandise would instead be dependent on the number of Gym Badges that have been won.