Identifying Diode Symbol and Layout on Printed Circuit Board

There are many types of diodes, it is being used as a voltage and current  rectifier,stabilizer,converter.
In electronics, a diode is a two-terminal electronic component called Anode andCathode that conducts electric current in only one direction.
The below picture shows a diode and a zener diode, A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".
It is being labeled in letter "V" means voltage rectifier in a circuit the mounting code and the product code.



A simple brief explanation,
Stabilize current and voltage - to maintain the flow of a current and voltage,
Rectifier - disallow unwanted current or voltage and purify it,
Converter - converts from AC voltage to DC voltage.

Light Emitting Diode
It is also a diode that can produce light. its like a light bulb.
A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting.
It also have Anode and Cathode terminal leads.



The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

PhotoDiode Photo Cell
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.
Meaning it has the ability to convert light energy into a voltage or current.
In mobile phone circuit it is being used as a sensor for a certain application of the device like for example, it switch the Camera Flash off when during daytime or when it detects that there is a light source on that certain area.
Photo Diode is widely used in a solar panels to produce free power energy from the sun light.

Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device.