Here is a little write up courtesy of Japan-Guide.com
"Natural hot springs (onsen) are numerous and highly popular across Japan. Every region of the country has its share of hot springs and resort towns, which come with them.
Hot spring baths come in many varieties, indoors and outdoors, gender separated and mixed, developed and undeveloped. A visit to an onsen is a highly recommended experience to any visitor of Japan."
A very important fact that is omitted in the above reference is that bathing suits are not allowed unless specified. We do frequent the Yunessan which is a "bathing suit" facility. It has more of an outdoor water park atmosphere than a traditional onsen.
This video is of a more traditional onsen.
(I can't get it to link...sorry you'll have to "cut and paste")
Here are my boys at the Yunessan...
Here is my Aunt Sue Sue and myself at a traditional onsen in Izu.
We love this silly picture! The hats are to keep you dry when it rains! What? Dry????!!!