For an historical perspective, you ought read on George Westinghouse and Thomas Edison. Edison promoted DC current systems for municipal lighting grids. Westinghouse promoted AC current systems. Westinghouse won. DC does not travel far on transmission lines before voltage losses mount. AC can travel quite some distance without degrading.
Modern electronic devices employing semi-conductors need DC current to work, but connect to AC line current. The power supply steps the voltage down to a practical level and rectifies it, that is, converts it to DC.
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AC power crosses between postitve and negative on each terminal, so on part of a second it will be positive and then it will change again, whereas DC is always either positive or negative on one pin, not alternating between them.
AC stands for alternating current. DC is direct current.
Alternating current comes in any number of waveforms and frequencies, but it always alternates direction. So it flows back and forth, rather than from point A to point B.
AC is useful for several reasons:
1. It has a lower arc-over potential compared with equivalent DC.
2. Changes in current produce magnetic fields. Changing magnetic fields can induce current. So with the use of a transformer, AC current can easily be stepped up or down in voltage.
3. Some motors are designed to run directly from AC current
4. Electric generators produce AC current.