The springs are split up into men's area and women's area.  In the past the men's area was considerably larger than the women's area.  However, in recent times many hot spring resorts have a larger women area.  This is because the average person taking a trip or vacation these days tends to be female.  Some hot spring resorts rotate the sex of the areas every day.  

Some hot springs have coed baths, but they are not that common. 

Visiting a hot spring in Japan is a must. Men and Women baths are seperated into different areas.

Hot springs which are not from a natural source, but are heated artificially are called sentou.  A sentou can be just as relaxing as an onsen with the exception that the smell of sulfur is not present. 

You WASH my back, I'll WASH yours!? 
While washing prior to entering the springs it would not be strange if someone offers to wash your back.  This normally doesn't happen out of the blue, but the person you have struck up a conversation with just might offer.  While in America the offer might be taken the wrong way, in Japan it is just a common courtesy. 

If you have the chance, I would highly recommend that you visit a Japanese hot springs, it is quite worth it. 

Glossary of Terms