In Japan, "bathroom" (called furo) in home usually consists of a bathtub and a separate space to wash. People can relax in the tub after washing body and hair in the washing space.
In many households, especially if living with family members, people do not refill the bathtub for every person, but instead keep the same water in until the last person takes a bath. To keep the water clean, you need to wash yourself thoroughly before soaking in the bathtub. In that way, remaining water can be used for laundry, or other needs in case of emergency such as earthquake.
In Japan, bathrooms tends to get mildewed due to high humidity. Regular cleaning can prevent this.

You can use paid public baths called sento if you do not have a bathroom in your home. Sento has separate rooms for males and females, each consisting of a big common tub and large washing space. When using sento, you must bring towels, soaps and other amenities for yourself (wash-basins are provided). Sharing a common tub with other people, you must always wash your body first before soaking, so that everyone can feel comfortable bathing. Available times and fees can vary slightly depending on the establishment. Usually they are open from around 15:00 to 23:00, with the fee around 400 yen for one adult.
In some places called "super sento" and "kenko rando (health spa)," fees can be higher (somewhere around 800-2000 yen) as they have more facilities such as variety tubs and saunas.
People with tattoos are not admitted in most of these places.