When talking about electrical current, AC stands for "alternating current" and DC stands for "direct current." Alternating current is the type of current that comes from a power plant and outlet, in which the current changes directions rapidly. Direct current is the type of current that comes from a battery, where the current is always flowing in the same direction.
Electrical current is simply the transfer of energy by sending electrons through a conductor. The electrons can be sent through the conductor, which is usually a wire, in two different ways. The first way is through alternating current, and the second is through direct current.
Alternating current means that the direction of the current changes back and forth rapidly. Alternating current is the safest way to transfer energy over large distances, and is what is used by the national grid to transport energy from a power plant to buildings. Using a transformer in conjunction with alternating current saves a lot of energy because transformers can easily change the voltage of the power coming through a wire. This means that the power can be divided and sent out into smaller voltages.
Direct current is the type of current that comes from a solar cell or battery. The positive and negative terminals of these power sources never change, so the current runs in the same direction the entire time. Direct current does not allow for power to be efficiently transported over large distances.