Capacitor
Measured in farads (F), usually µF or nF
A capacitor stores a small amount of electrical charge and releases it quickly.
Electrolytic capacitors have + and − legs (the stripe is −) and can burst if connected backwards. Ceramic ones (like 104 = 100 nF) have no direction.
They block steady DC but let changing signals through, which is why they're used for filtering and timing.
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Everything on this subject: Capacitors, Simplified