AC current alternates between a positive and a negative value over a time period. AC is constantly reversing direction. AC is almost universally used to send electricity over distances because transformers can be used to boost it to very high voltages at relatively low amperages. This reduces the losses caused by the heating of the high voltage transmission wires and results in more of the power arriving at its destination. Once the power reaches the end of the trip, other transformers reduce it to whatever voltage is required.
Most electronic equipment operates on DC power, at relatively low voltages.
The job of conditioning the electricity into the type and values needed is the work of the power supply, which we will take a closer look at next. There is a lot more we could go into such as electrical resistance, measured in ohms, and electrical storage, or capacitance measured in farads, but for the most part they are beyond the needs of our discussion here, and I can sense your eyes starting to glaze over. All computers need a source of electrical power. Our portable computers rely on battery power when an electrical outlet is not handy. Our desktop machines use a power supply, which really doesn't supply the power, but converts the power the electric company delivers to our homes and businesses into the form our computers use.