Definition
Static electricity is electric charge that sits still on the surface of an object instead of flowing. Unlike the current that leaves a power plant and keeps moving through wires, static charge is trapped in one spot and just piles up.
Why does rubbing two things together create electricity?
Everything is made of tiny atoms. At the center of an atom is a nucleus with a positive charge (+). Around it zip light, negatively charged electrons (-). Normally, the number of protons (+) in the nucleus equals the number of electrons (-), so the atom is neutral overall.
But when two different materials rub together, light electrons can hop from one object to the other. The object that gains electrons ends up with extra negative charge (-). The object that loses them ends up with extra positive charge (+). This imbalance caused by the movement of electrons is static electricity.
The same thing happens when you comb your hair with a plastic comb and it flies out in every direction. Your hairs pick up the same charge, so they push each other away.
A closer look: why does it zap?
Here's a closer look. When another object comes near, the trapped charge pushes or pulls on that object's electrons. This is called electrostatic induction. As the built-up charge grows, it eventually punches through the air and leaps to the other side in an instant.
That sharp zap you feel when you grab a metal doorknob in winter is a huge number of electrons jumping between your hand and the metal all at once. This sudden rush of electricity is called a discharge.
Lightning is just a giant static discharge, too. Ice particles in a cloud bump into each other, and charge separates into upper and lower regions. When the difference gets too big, electricity bursts through within the cloud, between clouds, or between a cloud and the ground. It's one enormous spark that flows in an instant.
Why is it quiet in summer but a pest in winter?
Static hardly ever shows up in summer, so why does it go wild in dry winter air? The secret is moisture in the air, meaning water molecules. When it's humid, a very thin film of water forms on surfaces like clothes and skin, and that film carries charge away fairly well.
When humidity is high, as in summer, charge trying to build up on your body slowly leaks away through that watery film. In winter, the air is bone dry, so electrons can't find an escape route and keep piling up on your body and clothes.
That's why rubbing moisturizing lotion on your hands or raising indoor humidity to 50-60% helps a lot. The moisture acts like stepping stones that let electrons slip away, so shocks get much weaker. Using fabric softener in the wash works too. It makes fibers smoother, which cuts down on friction and helps charge drain off.
Common misconceptions
Static electricity has very low voltage, so it's just a quick little sting.
The voltage of static electricity is actually very high, usually thousands to tens of thousands of volts (V). It just lasts far less than the blink of an eye and carries so little charge that it isn't dangerous to the human body.
Where you meet it
When you take off a knit sweater in winter, you may hear a crackle and see tiny sparks in a dark room.
Rub a balloon on your clothes and hold it near your head, and your hair clings to the balloon.
Plastic kitchen wrap clings to a bowl partly because of the static charge created as you pull it off the roll. Attraction between molecules also plays a part.
In one sentence
Static electricity is charge, moved by friction, that gets trapped on an object. In dry conditions it can discharge all at once and give you a sharp zap.