Most Japanese families have a bath (generally bath tub and space next to it to wash). In general bath and toilet are in separated rooms. A long time ago most families, especially in the city, didn’t have their own bath and people went to a public bath(sento/銭湯). In the country more family had baths, usually a detached building next to the toilet in the garden.
Let’s talk about the present day.
Japanese baths can generally be divided into 3 types: the private family bath, the public bath(sento/銭湯) and the hot spring bath(Onsen/温泉).
In all three types people share the same bath tub water, unlike a western bath, therefore you wash yourself before jumping into the water. Considering others and trying not to make the water dirty for those taking a bath later is very, very important. So the bath tub is not a place to wash your body but to soak and relax after getting clean.
Public baths and hot spring baths are usually bigger and communal. Some might even look like pool to you, but I advise you not to swim or splash unless you are all alone in the bath. I also advise not to wear swim suites even if you are embarrassed to be naked with strangers – the others are naked as well. Most baths are separated by gender. Unfortunately only a Japanese sign indicates which is which in most places, 男(male) usually blue or a dark colour sign and 女(female) usually red or pink.
Usually take a small towel or flannel to wash your body before getting into the bath, avoid dipping it into bath water, especially at hot springs, because it causes the water to get cloudy.
There might be other rules that I can’t think of right now because I take them for granted. I would be very happy if you let me know anything you think I’ve forgotten or ought to mention as well as anything you’d like to check or find out about.