AC reverses the current in periods, and the current flows in 2 directions.
DC only flows in one direction and is constant.
AC is typically used to transfer power over long distances from a generator to your home, stepping it up after it's generated to send it over the distance, then stepping it back down so that it is usable in your home.
DC is used typically inside of devices, as it sends a certain amount of electricity through the wires.
Nicolas Tesla, was ripped off by Edison to replace his DC stations with AC instead. ^^
