Field Effect Transistors (FETs) are semiconductor devices that control the flow of electrical current using an electric field. They have three main terminals: the source, the drain, and the gate. By applying a voltage to the gate, the conductivity between the source and drain can be modulated, allowing FETs to act as switches or amplifiers in electronic circuits. FETs come in various types, including JFETs (Junction FETs) and MOSFETs (Metal-Oxide-Semiconductor FETs). They are widely used in applications such as amplifiers, oscillators, and digital circuits due to their high input impedance and low power consumption.