[N.B: The product may have a slight difference what is written in the specifications. This depends on the supplier.]
Piezo elements come in handy when you need to detect vibration or a knock. You can use these for tap or knock sensors pretty easily by reading the voltage on the output. They can also be used for a very small audio transducer such as a buzzer. This basic Piezo Element measures 20mm across and is a great vibration sensor. Attach the round disc to the object that will be knocked on such as a door and measure the voltage across the two wires. Piezo elements can also produce sound when a voltage is applied across them. They convert between electricity and vibration in both ways; Vibration applied at the round end can be detected as a voltage across the wire leads and inversely, voltage applied across the wire leads causes the round end to vibrate.