This is a 16MHz quartz crystal oscillator. It's used in computers and programming boards such as an Arduino or Raspberry Pi. It uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency. It's main function is to help digital devices, such as Quartz wristwatches, to keep track of time very precisely.
The lenght of the metal housing of this crystal is about 11mm (roughly 0.43').
This particular one, is the most commonly used in circuits that you can create yourself, but there are also ones that are way smaller to fit on smaller pcb's and programming boards, such as the Arduino Micro.