An onsen is a Japanese hot spring. The onsen plays an important role in Japanese culture, providing (socially) institutionalised relief from the pressures of the contemporary Japanese twelve-hour work ethic and a chance for Japanese to break down the hierarchal nature of society through the mutual nakedness of skinship.
Ideally, they should be outdoors, use water directly extracted from a natural volcanic spring, and they are often embellished with (or, in the cheaper varieties, replaced by) a wide variety of extravagant spa baths, artificial waterfalls and saunas. The essential difference between an onsen and a sento (communal bath house) is that the water in an onsen must be volcanic spring in origin, even if reheated, whereas a sento may use ordinary heated water. Onsen water is often thought to have healing powers according to its mineral properties and onsens often have several different baths, each augmented by the addition of different minerals or the composition of the tub.
A key feature of the onsen is that as well as mere bathing facilities, there should be accommodation, extravagant cooking and all manner of relaxing pastimes - massages, aromatherapy, relaxation rooms and comfortable surrounds. Essentially, the onsen should be the diametric opposite of everything in normal, hectic day-to-day Japanese life.