A transistor is a key component that can allow different things to be hooked up to the BeagleBone over the expansion headers. Transistors allow things that need different voltages or more current to be controlled. A transistor has three terminals. The application of voltage or current to one terminal allows the current or voltage between the other two terminals to be controlled. This control property allows a very small signal to control a much larger signal. This has many implications for analog signals but we will mainly focus on transistors and digital signals. Other texts might refer to the transistor as a switch.
Details for the selection of transistors can be very complicated. We will go through a simplified description. Transistors can be divided into two general categories, namely, bipolar transistors and Field Effect Transistors (FETs).