Capacitors are used in most electronic circuits today. They come in a variety of the shapes and sizes. The standard through-hole capacitors that have been in use for most of the last century are typically ceramic or electrolytic. [There are also tantalum capacitors, film capacitors and variable capacitors.]
Electrolytic capacitors have one long and one short leg and are polarized, meaning that they need to be connected within a circuit so that the positive electricity flows in through the positive leg.
While the thru-hole capacitors are unlikely to fall out of fashion in the near future, circuitry is shrinking at a rapid pace. The capacitors (and other components) used in most contemporary circuit boards are what are known as surface mount devices (SMDs). On this Flora circuit board (pictured below and placed next to a quarter for comparison), the surface mount devices are the size of Washington's eye.
As Michael Faraday discovered, capacitance works by creating opposite charges on either side of a dielectric material. [see the cat analogy]. Capacitance is measured in units called farads, named so in honor of Faraday.
The Leyden Jar Project sculpture uses a MPR121 capacitance touch chip to detect changes in the capacitance around it. Your hand & your body are electrically conductive, so when you touch the electrodes (the gold covered jars), you are altering the amount of capacitance. Your smart phone works exactly the same way, reading the capacitance of your touch as you move your finger over the glass screen.
Another device that measures and responds to changes in environmental capacitance is the Theremin. This early twentieth century electronic instrument senses the proximity of large electrically conductive objects in its presence (i.e. your body). Many musicians have made use of the eerie sci-fi sound of the Theremin over the years, including the Beach Boys in their song Good Vibrations, the Pixies in Velouria, and The Flaming Lips’ Race for the Prize. Check out the google doodle in commemoration of Clara Rockmore's 105 birthday, one of the great classical musicians to use Leon Theremin's instrument.