There are four types of electron emissions.

1. **Thermionic Emission**:
- Thermionic emission occurs when electrons are emitted from a heated surface.
- When a metal is heated to a high temperature, the thermal energy causes some of the electrons in the metal to gain enough energy to overcome the attractive forces of the metal ions and escape into the surrounding space.
- This emission is used in devices like vacuum tubes and cathode ray tubes.

2. **Field Emission**:
- Field emission occurs when a strong electric field is applied to a metal surface, causing electrons to be emitted.
- In this type of emission, the electric field at the surface of the metal is so strong that it can pull the electrons out of the metal even without heating it.
- Field emission is used in devices like field emission displays and electron microscopes.

3. **Photoelectric Emission**:
- Photoelectric emission occurs when light or other electromagnetic radiation is incident on a surface and causes the emission of electrons.
- When photons of sufficient energy strike a material, they transfer their energy to electrons, allowing them to overcome the binding forces and escape from the material.
- The phenomenon of photoelectric emission is the basis of devices like solar cells and photomultiplier tubes.

4. **Secondary Emission**:
- Secondary emission occurs when a fast-moving electron collides with a material, causing the emission of additional electrons.
- When an energetic electron strikes a material, it can transfer enough energy to other electrons in the material to cause them to be emitted.
- Secondary emission is utilized in devices like photomultiplier tubes and image intensifiers.

In conclusion, there are four types of electron emissions: thermionic emission, field emission, photoelectric emission, and secondary emission. Each type occurs under different conditions and has its own applications in various devices and technologies.