[Nyororin]

Mainly it really is only for relaxation. After you`ve bathed well, get in and enjoy the heat.
You never put soap, towels etc into the onsen (unless you`re on TV and covering your private spots ;D ), and it`s best to try to avoid getting your hair into the water also.

Basically, you want to sit and soak. Onsen are generally quite hot, plus the water itself has minerals, so the heat feels quite different from that you`d experience in a home bath. Not to mention that a lot are outside where the contrast between the cool air and the hot bath is really striking.