AC current is a specific type of electric current in which the direction of the current's flow is reversed, or alternated, on a regular basis. Direct current is no different electrically from alternating current except for the fact that it flows in the same direction at all times. Alternating current was chosen early in the 20th century as the North American standard because it presented fewer risks and promised higher reliability than competing DC systems of the day. Many of DC's deficiencies were later corrected, but not until a substantial North American infrastructure had already been developed. DC is the European standard.
Electric power distribution requires a circuit, usually represented as two wires leading to a device that uses electricity. In AC current, one wire is negative and the other is either is positive or neutral (ground). The two wires take turns at sending electricity. In North America, AC current uses a standard "rhythm" in which each side gets its turn 60 times each second, thus the 60Hz designation given to standard AC current. This switching of polarity takes the form of a rhythmic pulse in the electrical current that occurs within the normal audible range. This is why you can actually hear this rhythm in circuits such as fluorescent lighting ballasts and audio equipment as a low buzzing tone. This buzz is referred to as "sixty cycle hum". Prior to the 1970s, two AC power schemes were used in North America. One offered energy at 45-50Hz, the other at 60Hz. "Fifty-cycle power", occasionally referred to as "rural power", is now obsolete and the 60Hz standard is now used throughout North America.
In DC circuits, the electricity is always the same polarity, which means that in a two-wire circuit, one "wire", or side of the circuit, is always negative, and the negative side is always the one that sends the electricity. There is no hum because there is no cyclic change in current flow. DC current is more effective for long-distance, high-voltage transmission because it results in less energy lost in transmission, but the cost of converting DC current to AC is relatively high, so DC is typically cost-effective only for long-distance transmission.
Electrical devices that convert electricity directly into other forms of energy can operate just as effectively from AC current as from DC. Lightbulbs and heating elements don't care whether their energy is supplied by AC or DC current. However, nearly all modern electronic devices require direct current for their operation. Alternating current is still used to deliver electricity to the device, and a transformer is included with these devices to convert AC power to DC power (usually at much lower than the supplied voltage) so that electronic devices can use it.
See also:power, ground, transformer, ballast, incandescent lamp, fluorescent lamp