Oh, interesting!
Apparently Cream of Tartar is Potassium bitartrate, a potassium acid salt. And baking powder is basically an acid (Cream of Tartar for example), mixed with a base (Baking Soda, or Sodium Bicarbonate) and a desiccant like corn starch to prevent the two from reacting by getting wet.
When they do mix, an acid/base reaction occurs releasing CO2, which is why it's useful in baking because it makes pockets in the dough.
Apparently cream of tartar was a naturally occurring byproduct of wine making, and has been known for thousands of years, but was only used for baking in the last few hundred years.
The word "tartar" seems to be basically as old as wine making. The latin word "tartarum" has its roots in Byzantine Greek. But, it doesn't seem to be connected etymologically to tartar sauce.