What is the difference between a Steambath and a Sauna?

The temperature and humidity levels differ along with the room construction.

Steam is wet with a temperature generally ranging between 110 - 120+° F on average and close to 100% relative humidity. 
Room construction for steam units is generally ceramic tile, stone, or acrylic

Sauna is dry with average temperatures ranging between 165 - 180+° F and with 10 - 15% humidity.
Room construction for saunas are generally cedar or redwood.