Transistors are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. Transistors are manufactured in different shapes but they have three leads (legs).
A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector). The device can be characterized as a current amplifier, having many applications for amplification and switching.