A capacitor is important in nearly every circuit.

A capacitor is a pair of plates with an insulating layer between them. You can create a homemade capacitor by layering a sheet of aluminum foil, wax paper, and another sheet of foil. If you connect leads to the 2 ‘plates’ and roll them up, you have created a basic capacitor. There is no direct electrical connection between the plates.

How they work
As one plate has voltage applied, it charges one plate in an electric field.

A capacitor is measured in farads. A farad is a massive amount of capacitance. You will typically see values in microfarads (1 millionth of a farad) and picofarads (1 trillionth of a farad).

Capacitors are important because they resist DC but allow AC. Combine these with inductors (resist AC but allow DC), and you can find the basis of filters, oscillators, and tuned circuits.
You can read more about Inductors here: WHAT IS AN INDUCTOR

Types of Capacitors
Ceramic Disc
Non polazied
Small values – can be high voltage
Electrolytic
Poarized
High values, small voltages
Film
Variable
Paper/Wax (Vintage)