There are two types of electrical current: direct current (also known as DC) and alternating current (also known AC). Direct current is unidirectional. That is, it flows in only one direction and hence does not change directions. We commonly use direct current in battery operated equipment such as flash lights, watches, mobile phones, etc.
Alternating current or AC is the current that changes directions back and forth at regular intervals. In fact, it changes directions 60 times per second in electrical supplies in the US. That is why the frequency of AC in the US is 60 Hz. For many regions of the world, the frequency of AC is 50 Hz (or 50 cycles per second). Most of our household products work on AC. AC is also useful because of low transmission losses, as compared to DC.
Hope this helps.