A crystal oscillator is an electromechanical device, it's also an electronic oscillator circuit. The main purpose of an oscillator is to make use of a quartz crystal as a frequency. Crystal oscillators use the mechanical resonance of a vibrating crystal, which is made up of a piezoelectric material. This process allows the oscillator to create and obtain an electric signal with a high precision frequency.

Where Crystal Oscillators are used?

This type of frequency is mostly used to keep track of time within a wristwatch or a digital integrated circuit, to provide a stable clock signal. This is where this signal gained it's commonly referred to as "clock signal". This clock signal is used to synchronise the operation of the other electronic devices that make up the system the oscillator is housed in. An oscillator is also used to help stabilise frequencies for radio transmitters and receivers, used in clocks (clock oscillator), computers and cell phones. Quartz crystal can also be found inside test and measurement equipment such as counters, signal generators and oscilloscopes.

A little more information on Crystal Oscillators

Crystal oscillators are known to be superior to ceramic resonators, this is for a multitude of reasons. They offer higher stability, they're higher quality and are considerably lower in cost and more compact in size.
Quartz crystals are manufactured for frequencies that range from a few tens of kilohertz all the way up to hundreds of megahertz.

Types of Crystal Oscillators

TCXO - These are used when a temperature stability is needed and can't be attained through a standard crystal oscillator (XO) or a voltage-controlled crystal oscillator (VCXO).
OCXO - This is an oven-controlled crystal oscillator, a type of oscillator that achieves the highest frequency stability that a quartz crystal can. They are designed to operate inside a temperature-controlled oven, they are physically larger and require more power to run.
VCXO - This is a voltage-controlled crystal oscillator, it controls the change in crystal output by using a DC (direct current) voltage.