What is a transistor?

A transistor is a type of semiconductor device that can be used as both an amplifier or as an electronic switch.

Reference Practical Electronics for Inventors

What are the different types of transistors?

Bipolar Junction Transistor (BJT) come in both NPN and PNP forms

• Insulated Gate Bipolar Transistor

MOSFETs come in many flavors, and all have both N-channel and P-channel forms

• UniJunction Transistors (UJT)
• Junction Field Effect Transistors (JFET)
• Depletion Mode MOSFET
• Enhancement Mode MOSFET
• Power MOSFET (HEXFET)

What common situations might I need a transistor?

How do I choose a transistor from the thousands of options?