What is an onsen?

An onsen is a natural, mineral rich hot spring where Japanese go to soak away their aches and pains or just to get away and relax.
Bathing at onsen is a centuries old tradition in Japan that shows no sign of abating, in fact, it is probably more popular than ever.

Onsen can be simple open-air pools near rolling rivers, little rickety huts on lonely mountains, gorgeous traditional inns (ryokan) or modern sprawling complexes.
Some public onsen are free and others you pay a fee ranging upwards from a few hundred yen. Or you can stay at an hotel, ryokan or pension and have their private baths at your disposal all day (for many this means 24 hours).

Onsen have many different chemical compositions (depending on the area) and each onsen has a list of what ailments the water is effective against.
Many people swear by the effectiveness of the properties of onsen - whether bathed in or drunk (be sure to check if the water is supposed to be drunk first!).

Most onsen now have separate male and female bathing areas, but you can still find some that have mixed bathing - some require bathing suits and others not.

One problem to be aware of is that many onsen do not allow people with tattoos to bathe, so be sure to check this out with them first and avoid problems later.
What is a ryokan?
A ryokan is a traditional style Japanese inn. Though some of them are decidedly modern, they still manage to retain their traditional atmosphere with kimono clad female staff to greet you and guests wandering around in lightweight cotton kimonos (called 'yukata' - usually provided by the ryokan).
Many of them have their own onsen - though the water may not always be the usual mineral rich variety found in registered onsen. Sometimes the water is heated mechanically rather than naturally, but a nice hot soak at a nice ryokan is an added bonus.
They all serve traditional Japanese meals and usually try to adjust their menu to whatever foods are in season for that area at the time.
The guest rooms are pure Japanese with 'tatami' (straw) matting, shoji (paper) sliding screens with low tables and a scroll painting or calligraphy hanging in a small alcove. Close the door and you step back in time - except for the giveaways like the TV, tea maker and electric lights.
Your bed will usually be a futon (thin cotton mattress) that goes on the floor and is stored in a cupboard in your room - it will be nicely laid out for you by the staff before bed time. These usually have a foam or cotton under mattress to give you a bit more comfort if you need it.