A MOSFET is short for a Metal Oxide Semiconductor Field Effect Transistor. The basic working principle is that when a voltage (exceeding the threshold volatage) is applied to the Gate pin it activates the MOSFET by either reducing or increasing the conductance across the Source and Drain pins. The Metal Oxide part of the name comes from the material, Silicon Dioxide, used between the semiconductor material.

Types of MOSFETs

• Enhancement Mode:

• No voltage across gate terminal: device does not conduct.
• Applying a voltage increases the width of the conductance channel.
• Maximum voltage across the gate terminal: maximum conductance.

• Depletion Mode:

• No voltage across gate terminal: maximum conductance.
• Applying a voltage decreases the width of the conductance channel.
• Any positive or negative voltage across the gate terminal: decreased conductivity.

Both of these have N-Type and P-Type variants depending on their structure:

• N-Type:

  The Gate pin is connected to N-Type semiconductor, so a positive gate voltage attracts electrons to form a N-Type channel that allows for conductance.
• Positive voltage applied to gate.
• Current flows from Drain to Source.

• P-Type:

  The Gate pin is connected to P-Type semiconductor, so a negative gate voltage attracts holes to form a P-Type channel that allows for conductance.
• Negative voltage applied to gate.
• Current flows from Source to Drain.

Watch the video for more diagrams.

3 pins of a MOSFET

• Gate: Applying a voltage to the gate pain activates (increasing or decreasing conductance - keep reading, I explain this later; the video below covers it too) a MOSFET. 
• Drain: Current flows from Drain to Source in N-Type MOSFETs
• Source: Current flows from Source to Drain in P-Type MOSFETs

Applications

• Switching: MOSFETs are widely used in digital circuits as switches, turning on and off rapidly to control signals in microprocessors, memory, and other digital logic circuits.
• Amplification: MOSFETs can amplify analog signals in audio devices, RF circuits, and other analog applications by controlling the amount of current flowing through the channel.
• Power Management: Power MOSFETs are used in power supplies, motor controllers, and converters due to their ability to handle high currents and voltages with minimal loss.