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.
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.