A bathtub is a bathroom fixture, a tub that you can fill with water for bathing. Some people like to soak in a bathtub at the end of a long day.
Many bathtubs are made of ceramic or porcelain, and they're large enough for an adult to lie down in at least partially submerged in water. Some bathrooms have only a bathtub, or only a shower, while others have both. In Britain, it's more common to call a bathtub simply a bath. During Prohibition in 1920s America, "bathtub gin" referred to any kind of home-brewed alcohol, whether it was actually made in a bathtub or not.