Static shocks are caused by the transfer of electrical charge from one object to another. Every physical thing we experience is made of electrically charged matter, which is either "positive" or "negative". However, usually the amount of positive charge is the same as the amount of negative charge, so the total charge is zero. It is like adding '1's and then subtracting the same number of '1's, for example: 1+1+1+1+1-1-1-1-1-1=0.

When things touch each other some charges can move from one object to another. When you walk on a carpeted floor, negative charges move into your body. Because door knobs are usually made of metal (which conducts electrical charges very well), when you touch a door knob these extra electrical charges leave your body and go to the door knob. This is what gives you an electrical shock. But don't worry, although it is electricity flowing through your body, these types of static shocks aren't dangerous because the amount of charged particles flowing between you and the door knob is quite small.