Piezo element, piezo beeper, piezo sensor, piezo crystal, piezo microphone....
It's all about piezo, and there is no rule how to use the names.
The photo you uploaded explains what you have very well!
A normal speaker of 8 Ohms must have a 100 Ohm series resistors.
But also for a piezo, it is better to add a 100 Ohm series resistor, to protect the output pin of the Arduino with higher frequencies.
You can use a 1 MOhm in parallel and add a 100 Ohm series resistor. That way you can detect a knock and play a tone with the same piezo element.
A library "toneAC" doubles the volume.http://arduino.cc/forum/index.php?topic=142097.0
You still need that 100 Ohm series resistor.
For more volume, you could use transistors or a driver ic and more voltage like 12V.
Also the way the piezo element is mechanically placed into a case, could make the sound a lot louder.