A capacitor stores energy in an electric field between two conductive plates separated by a dielectric (insulator). Its capacitance — measured in farads (F) — depends on plate area, plate spacing, and the dielectric material. The fundamental relationship is Q = CV, where Q is charge in coulombs, C is capacitance, and V is voltage.

In practice, you'll encounter several distinct capacitor families, each with trade-offs:

• Ceramic (MLCC): Tiny, cheap, low ESR, non-polarized. Available from picofarads up to ~100 µF. Class 1 types (C0G/NP0) are extremely stable — use them in filters and timing circuits. Class 2 types (X5R, X7R) offer more capacitance but lose up to 50% of their rated value at full DC bias. The most common capacitor in modern electronics.
• Electrolytic (Aluminum): Polarized, high capacitance (1 µF to tens of thousands of µF), but higher ESR and limited lifespan (typically 2,000–10,000 hours at rated temperature). Used for bulk power supply filtering. Connect them backwards and they can violently fail.
• Tantalum: Polarized, lower ESR than aluminum electrolytics, more stable, but they fail short-circuit — meaning a voltage spike can cause them to catch fire. Derate to 50% of rated voltage for reliability.
• Film (Polyester, Polypropylene): Non-polarized, excellent stability, very low losses. Used in audio crossover networks, AC-rated safety capacitors (X/Y class), and snubber circuits. Physically larger than ceramics for equivalent capacitance.

Real-world example: A switching power supply's output stage typically uses a combination: bulk aluminum electrolytics (e.g., 470 µF) to handle low-frequency ripple, paralleled with small ceramics (e.g., 10 µF X7R + 100 nF C0G) to suppress high-frequency noise. Each type handles a different part of the frequency spectrum because ESR and capacitance behave differently at different frequencies.

Quick calculation — RC time constant: A 10 kΩ resistor in series with a 100 µF capacitor gives τ = R × C = 10,000 × 0.0001 = 1 second. The capacitor reaches ~63% of the applied voltage after one time constant, and ~99% after 5τ (5 seconds). This is how simple timing circuits, debounce filters, and soft-start circuits work.

Rule of thumb for energy storage: E = ½CV². A 1,000 µF capacitor charged to 50 V stores ½ × 0.001 × 2500 = 1.25 joules — enough to give you a sharp zap, but nowhere near a battery. Doubling voltage quadruples stored energy, which is why high-voltage capacitors demand respect.