The oldest known design dates from the Greeks about 10 yrs A. D., and is known as Hero's Aeolipile (How you pronounce that, I am not sure). Here is a drawing of one from antiquity:
Apparently a few of these were made from bronze, and were simply curiousities. Heat from the fire below boils water in the bronze reservior. Steam from the boiling water is carried up in pipes to a hollow ball at the top. The pipes that carry the steam up to the ball support it, but also allow the ball to rotate freely on the axis. The steam is then allowed to escape from the ball via two nozzles, spinning the ball.
A couple of interesting things to note about this design:
- This design would not be useful as anything other than a curiousity because it has no output shaft.
- The very first steam turbine design is a reaction turbine.
Item #2 is interesting because although Issac Newton didn't explain the Third Law of Motion until 1687, the process was understood long before then! Newton's third law is usually paraphrased as "For every action there is an equal and opposite reaction", thus the term "reaction" turbine.
Reaction turbines achieve movement (and actual work) by squirting steam through nozzles, like our Aeolipile above. The steam squirts out, and the reaction force causes the ball to spin in the opposite direction from the steam jet.
The other type of turbine is called an impulse turbine. In this design, steam is directed at a set of blades on a shaft and bounced off of them. This provides an "impulse" to move. Below is a simple diagram showing the difference between the two basic designs.