The Store is essentially just a regular Yall-Mart, but with tons of food. There is usually over 60 food in the Untouched Store. The store still has the same storage rooms and such as a regular Yall-Mart.
The zombie size will be Sardine-Packed, and if you're in the late game stages, it might be Death Swarmed, but if you have the right weapons, the rewards will be worth it. Generally this location has a lot more zombies, although this depends on the mode.
The outbreak in this crowded grocery store happened so fast that the panicked crowd turned into zombies before the place could be looted. Many of the groceries are still on the shelves.