Direct current (DC) is electricity flowing in a constant direction, and possessing a voltage with constant polarity. DC is the kind of electricity made by for example by a battery (with definite positive and negative terminals). In the first volume of this book series, DC electric circuits are explored.
DC is not the only "kind" of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this "kind" of electricity is known as Alternating Current (AC). In this volume of the book series, electric circuits are explored where the direction of current switches back and forth.