What does a capacitor do in a circuit?

Capacitor structure: two conductive plates separated by an insulating material (the 'dielectric' — air, ceramic, plastic, electrolyte). When voltage is applied, positive charge accumulates on one plate and negative on the other; energy is stored in the electric field between them. Behavior in circuits: (1) DC: capacitor initially conducts as it charges, but once fully charged it blocks DC; like an open circuit at steady state; (2) AC: capacitor continuously charges and discharges as voltage alternates; appears as a frequency-dependent impedance (Xc = 1/(2πfC)); lower impedance at higher frequencies. Applications: (1) POWER SUPPLY FILTERING — large capacitors smooth ripple from rectified AC, providing cleaner DC; (2) DECOUPLING — small capacitors near IC power pins filter noise; (3) TIMING — capacitor charges through a resistor at a predictable rate (RC time constant = R × C); used in oscillators and timers (555 timer is famous); (4) AC COUPLING — passes AC signal between stages while blocking DC bias differences; (5) TUNING — variable capacitors in radio receivers select frequencies; (6) ENERGY STORAGE — large capacitors in flash photography, defibrillators, electric vehicles. Units: Farad (F) — large; typical practical values are microfarad (μF = 10⁻⁶ F), nanofarad (nF = 10⁻⁹), picofarad (pF = 10⁻¹²). Types: ceramic (small, stable), electrolytic (large capacitance, polarized — must be installed correctly), tantalum (compact, polarized), film (high quality, audio), supercapacitor (very large, energy storage). Safety: large capacitors store dangerous charge even after circuit is unplugged; discharge before working on equipment. CAUTION: electrolytic capacitors are polarized — backwards installation can cause failure, leakage, or explosion.