Skin Depth
The skin depth is defined as the depth below the surface of the conductor at which the current density decays to 1/e (about 0.37) of JS
AC can penetrate inside the conductor, but upto experimentally it is seen that this can happen upto certain extent. But AC goes on decreasing as defined in the definition.
It can not be observed with DC because this phenomenon is for electromagnetic wave.

AC and DC
Electricity flows in two ways, either in alternating current (AC) or in direct current (DC). The difference between AC and DC has to do with the direction in which the electrons flow.
In DC, the electrons flow steadily in a single direction, or “forward.” In AC, electrons keep switching directions, sometimes going “forwards” and then going “backwards”. AC electricity is the type of electricity commonly used in homes and businesses throughout the world. While direct current DC electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country. This is called the frequency and is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz). In India, value of frequency is 50 Hz.
AC electricity is created by an AC electric generator, which determines the frequency. What is special about AC electricity is that the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.
Difference between AC and DC electricity
Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire.
Electrons move direct with DC electricity
With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) terminal will cause the negative charged electrons to rush through the wire toward the positive charged side. The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal.
Electrons alternate directions in AC electricity
With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position.
The regular back-and-forth motion of the electrons in a wire when powered by AC electricity is periodic motion, similar to that of a pendulum. Because of this periodic motion of the electrons, the voltage and current follow a sine waveform, alternating between positive (+) and negative (−), as measured with a voltmeter or multimeter.
The major advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages.
Since high voltages are more efficient for sending electricity great distances, AC electricity has an advantage over DC. This is because the high voltages from the power station can be easily reduced to a safer voltage for use in the house.
Light bulbs
Many electrical devices—like light bulbs—only require that the electrons move. They don’t care if the electrons flow through the wire or simply move back-and-forth. Therefore, a light bulb can be used with either AC or DC electricity.

References:
1. http://www.school-for-champions.com/science/ac.htm
2. http://www.wikipedia.org