Crystals and oscillators are electronic components that generate precise frequency signals. Crystals refer to the quartz or other piezoelectric materials used in oscillators, while oscillators are the complete devices that include the crystal and additional circuitry to produce a stable frequency.

Function:
The primary function of a crystal oscillator is to provide a stable and accurate frequency reference for electronic systems. This is crucial for applications requiring precise timing, such as in communication systems, digital clocks, and microcontroller-based devices.

Applications:
1. Communication Systems: Crystal oscillators are used in radio frequency (RF) transmitters and receivers to maintain signal integrity and synchronization.
2. Computers and Servers: They provide the clock signal for processors and memory devices, ensuring data is processed and accessed at the correct rate.
3. Consumer Electronics: In devices like smartphones and TVs, oscillators help maintain the correct operation of digital circuits.
4. Automotive Systems: They are used in engine control units (ECUs) and infotainment systems for timing and synchronization.
5. Industrial Automation: For precise control of machinery and processes, ensuring reliable operation and coordination.

Selection Criteria:
1. Frequency Stability: The ability of the oscillator to maintain a constant frequency over time and under varying conditions.
2. Temperature Stability: How well the oscillator's frequency remains stable across a range of temperatures.
3. Frequency Range: The specific frequency required for the application.
4. Power Consumption: Important for battery-operated devices or systems where energy efficiency is critical.
5. Size and Form Factor: Depending on the space constraints of the device or system.
6. Cost: Balancing performance with budget considerations.
7. Supply Chain and Availability: Ensuring a reliable source of components, especially for long-term projects.

In summary, crystals and oscillators are essential for ensuring the accurate and reliable operation of electronic systems across a wide range of industries and applications. Their selection is based on a combination of technical specifications and practical considerations.