I performs two basic functions. It acts as a switch turning current on and off. It acts as a amplifier. This makes an output signal that is a magnified version of the input signal. More on transistors in later sections.
Transistors come in several sizes depending on their application.
Integrated Circuits, or ICs, are complex circuits inside one simple package. Silicon and metals are used to simulate resistors, capacitors, transistors, etc. It is a space saving miracle. These components come in a wide variety of packages and sizes. Their applications are as varied as their packages.