MOSFET
Metal-oxide-semiconductor field-effect transistor, e.g. IRFZ44N
A MOSFET is a transistor switched by voltage instead of current, so it can control big loads with almost no input current.
Legs: gate, drain, source. For 3.3 V or 5 V microcontrollers pick a 'logic-level' MOSFET, or it won't switch fully on.
Used for motors, LED strips and inside almost every power supply and computer chip.
Words to know
Everything on this subject: Transistors, MOSFETs and Relays, Simplified