TheTechMatterAnswer: Japan Travel & Internet survival guide

An onsen is a Japanese bath.
A rotemburo is an outdoor onsen.

An onsen is not used for washing but for relaxing. Should be considered a kind of sauna because the water temperature reaches nearly 42 degrees centigrade, so you can not stay too long in the tub.

There are two types of onsen depending on the water type used in the bath:
- of sulphurous thermal water (hot springs with different degrees of acidity)
- of mineral water (non-volcanic)

The onsen are:
- Public private structures (eg in your hotel and usable by all guests)
- Private in private structures (eg associated with your hotel room and usable only by you)
- Public in public structures (usable by anyone)

Many zones of Japan are volcanic and sulphurous water flows naturally from the earth (hot spring), thus in those areas were built public baths that exploit the thermal properties.

How we can use an onsen or rotemburo:
The bathroom is usually within a structure divided into three areas (more or less distinct) to be used in the following order:
- A first environment is the dressing room where you must leave all clothes. From this room you can proceed with a small towel covering private parts. Typically there is a set of safety boxes with a security key (which you can bring with you in the bath) for valuables storage.
- A second area is the area where we have to wash ourself. There are many small seats where you sit and wash with a shower and soaps. Warning: you must rinse thoroughly before proceeding to the next room
- The last environment is the  bath itself, in which you can enter covering your private parts with the small towel that you cannot wet (you could for example put it on head or in the tub border).
- After leaving the bath you have to return in the first environment and dry yourself with a large towel.

Here are some guidelines on these bathrooms:

The onsen as well as rotemburo can be public or private (inside of your hotel for example).
In public bath is usually not allowed to enter if you have any visible tattoos
The bathrooms are divided into male and female zone. The mixed baths are currently very rare and only private.
Never enter in the water before being washed.

Here’s an example of onsen:

and of rotemburo (Sai No Kawara park in Kusatsu Onsen):