Transistors are semiconductor devices that regulate current, amplify signals, and act as switches, forming the foundation of modern electronics.

The transistor has amplified the original signal. Since this transistor depends on an electric field, it's known as a Field Effect transistor. Even more on the Field Effect Transistor.

Take a look at this video as well for an intuitive explanation of the close cousin of the BJT, the field-effect transistor.

These transistors are very small in size and there can be millions and billions of transistors on one chip. The term nanometer in chips is referred to as the size of each individual transistor.