Gift shops (ギフトショップ) or souvenir shops are shops where you can buy memorabilia items for you or people back home. They are also popular among Japanese, as different prefecture have different types of limited goods.
You can find these shops at airports, near or sometimes even inside popular temples or shrines, as well as in train or metro stations and department stores. Depending on the location they will offer a variety of goods such as limited and local food, special charms, miniature buildings, fans, and many many more.
On some busy locations you can find dozens of gift shops next to each other with sometimes similar items but different prices. It is therefor recommended that when you find something you want to buy in one shop to look around for similar items in other gift shops, as some items may be cheaper there.
Some gift shops on special events or festivals are known to increase their prices, which is why you should try to find similar shops before or after said events, or even just go back to the same shop to buy the item at the normal price.