The Japanese are obsessed with baths and they like to indulge themselves in ways that to others might seem excessive. Visitors to Japan find some things puzzling so here's an "insider"s presentation of all things bath related.
Below is a photo, from here, of a Japanese bath in a typical household.
The area outside the bathtub is for squatting and washing yourself. Once you are done, you enter the bathtub filled with hot water and soak. For the Japanese the hot water is really hot, more than 40C/104F and quite often salts and minerals can be added for extra colour and smell. Don't dip your head into the water and before leaving the bathroom take another quick shower to cool down and wash away the salts/minerals. Japanese families traditionally go through all this litany on a daily basis, the children always waiting for the time to share the bath with their father who comes back home after a long and hard day's work. Quality family time together and bonding are perfectly fine but creepily enough about 10% of girls admit taking their daily bath with their father up until high-school! (source, here)
In many other countries around the world the waste of so much water for a 10-15 minute relaxing time would seem ridiculous but not in Japan where water is cheap and is heated by natural gas which is also cheap.
Note that the toilet is not in the bathroom but in a separate small room. One of the absolutely great things in Japan.
Sento is called the communal bath house where you pay to take a bath. Below is a pretty nice video, from here, on how to use a sento (ignore the Chinese music though).
The same rules apply to the onsen, the Japanese hot springs or spa. These are a big thing in Japan and if you are not visiting the country for only a short business trip you should definitely take the chance to enjoy an onsen. The good ones are located in really idyllic places, offer beautiful quite traditional facilities and unbelievable local food. They are pricey but worthy. If you're staying in Tokyo highly recommended are the onsen in Hakone, 70 mins from Tokyo. Staying for only one night would be enough.
Now, I've never entered a public bath or onsen. I don't want to get scarred for life taking a bath with other naked men and my tattoos would be a problem since many facilities refuse entrance to tat people because yakuza usually have aplenty. But there are onsen with private hot-spring baths in each room and they are fantastic.
Below are some onsen images from google.
And finally here's a short one of the lighter side of sento by the absolutely best Japanese comedians; The Drifters.
0:11 - Hi!
0:12 - Welcome!
0:51 - prepare the shampoo
01:01 - pour the water
01:05 - c'mon shampoo, shampoo
01:34 - next, we wash. First the hands.
02:12 - we dry your body.
02:24 - okay, we are done
02:29 - thank you very much
02:33 - what happened?
02:34 - fail?
02:35 - can't say the lines?
02:36 - this sucks!
02:55 - shampoo got in my eyes and I can't open them, you bastards. Water got into my ears too...
03:02 - what a cry-baby
03:10 - ok, let's go for the real take!
And last but not least you should never, ever, confuse the sento and the onsen with the sleazy and bizzare soapland facilities.