Transistor
BJT (NPN, PNP), e.g. BC547, 2N2222
A transistor is a switch or amplifier where a small current controls a much bigger one.
Its three legs are base, collector and emitter. An Arduino pin can drive the base through a resistor to switch a motor or a strip of LEDs.
The gain (hFE, often 100+) says how many times bigger the controlled current is than the base current.
Lessons
Everything on this subject: Transistors, MOSFETs and Relays, Simplified