DC stands for "Direct Current", which is a slightly incorrect name for electricity supplied at a constant voltage. If all you have is resistors (light bulbs, toasters, etc.) then the current will always flow in the same direction, hence the name direct current. AC stands for "Alternating Current", where the voltage changes from plus to minus and back again many times a second. In the United states, electricity is supplied by the power companies with its voltage changing from plus to minus and back sixty times a second. In Europe, it's fifty times a second.
Direct current is most commonly found in things that are powered by batteries. A battery's voltage does not change much with time, so it is an ideal source of DC electricity. Flashlights, cell phones, watches, radios, calculators and toys are all places where battery-powered DC is at work. Cars carry around big batteries for powering the starter motor and the headlights (and the radio and the windshield wipers and everything else).
Direct current is also found in electronic items that plug into the wall, such as computers, radios, and other items. These items all have a component called a power supply which converts the alternating current coming out of the wall socket to direct current of the appropriate voltage inside the appliance.
(republished on 07/13/06)