There are two types of current – DIRECT current (DC) and ALTERNATING current (AC).
Direct Current is unidirectional which means the flow of charges are only in one direction.
Alternating current is the one in which the direction of flow of electric charge reverses periodically. For understanding purpose let’s say Alternating Current starts from Zero, reaches maximum, decreases to Zero, goes to maximum in the negative direction, then returns again to zero. This interval of time taken by a single cycle is called a period. The number of cycles per second is called frequency and is measured in Hertz (Hz).
DC supply powers our alarm clocks, laptops, wrist watches, torch lights etc. Electrons move from one point to another in small amounts when you use batteries. Direct current offers a steady and safe flow of electrons while alternating current switches direction in a constant period of time.
Sources of DC are battery, solar cells etc whereas current generated by power stations and distributed to our houses are AC. When anything is plugged into your walls, you are using Alternating Current. It is lot more easier and cheaper to transmit Alternating current rather than Direct current. To learn why, read about electrical power transmission and distribution system works.
The voltage and frequency varies from country to country. For example, the US uses 120V 60Hz AC (meaning the direction of current alternates 60 times per second) whereas in Europe and India it is 230V 50Hz. While you bring an electrical appliance say a mobile charger from one country to another, it might require some special converter or adapter to allow the appliance or device to work properly.
Scientist Nikola Tesla worked on Alternating Currents and developed many high voltage systems which Edison felt was very dangerous. Today Tesla is best known for his contribution to the use of commercial electricity. George Westinghouse Jr was also a rival to Edison as he again insisted on developing systems based on Alternating Current.