Transistors are semiconductor devices used as switches, and to amplify currents. They are a key component in almost any electronic device.
There are two kinds, npn and pnp. The npn transistor has a p-type semiconductor between two n-type semiconductors. A pnp transistor has an n-type, between two p-types.
Each of the three regions has a terminal. The center region is always called the base. The outer regions are the collector, and the emitter. In the symbol, the emitter is the terminal with the arrow. always pointing to the negative material.
In a circuit, npn transistors can act as a switch. If the control switch is open, the depletion layer at one pn-junction is blocking current from flowing through the transistor and driving the load.
With a closed control switch, a small current flows through the emitter-base pn-junction. The base has only a limited number of charge carriers, so extra ones flow across the emitter-collector pn-junction, letting current operate the load. The transistor then operates as a low-resistance conductor. A small current through the base lets larger current flow across the emitter-collector junction. The transistor is then said to be turned on.