What is the function of a resistor in an electrical circuit?

Common electronic components and their functions: (1) RESISTOR — limits current and drops voltage; converts electrical energy to heat; color-coded with bands indicating resistance value and tolerance; measured in Ohms; (2) CAPACITOR — stores electrical charge temporarily; passes AC, blocks DC; used for smoothing power supplies, filtering signals, timing circuits; measured in Farads (typically μF or pF); (3) INDUCTOR — stores energy in magnetic field when current flows; resists changes in current; passes DC, restricts AC at high frequencies; measured in Henries; (4) DIODE — allows current in one direction only, blocks reverse direction; used for rectification (converting AC to DC), protection circuits; symbol: triangle pointing to a line; (5) LED (Light Emitting Diode) — diode that emits light when current flows; uses much less energy than incandescent bulbs; long-lasting; specific forward voltage needed; (6) TRANSISTOR — amplifier or switch; controls a large current with a small current/voltage; foundation of all modern electronics; BJT (bipolar junction transistor) and FET (field-effect transistor) types; (7) TRANSFORMER — transfers electrical energy between circuits via magnetic field; can step voltage up or down; works only with AC; primary and secondary windings; (8) FUSE — protective device that breaks the circuit when current exceeds a rated value (the metal element melts); single-use, must be replaced; (9) CIRCUIT BREAKER — like a fuse but resettable mechanically; common in residential breaker panels; (10) SWITCH — manually opens/closes circuit; (11) RELAY — electrically-controlled switch; small electrical signal controls a larger circuit; (12) BATTERY — DC voltage source. Resistor color codes (heavily tested on ASVAB EI): Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Four bands: first two are digits, third is multiplier (10^x), fourth is tolerance (Gold=5%, Silver=10%).