DC (Direct Current) always travels in the same direction. imagine a band saw. the blade is always moving in the same direction at the point where the work (cutting) is being done.
AC (Alternating current) essentially goes back and forth. imagine a jigsaw, reciprocating saw (sawzall) or hand saw. the blade moves back and forth at the point where the work is being done.
this is a pretty crude comparison, but it gives you a basic understanding of the behavior of the electricity.
there are advantages and disadvantages of both forms of travel:
DC is very simple. it can come from a battery, or a DC generator. DC motors are very simple: they have a winding and a magnet, and when DC is applied to the winding, with a mechanical switch for reversing the current every time the motor turns 180 degrees, the motor will run just fine. however, DC voltage cannot be "transformed" easily. most times, to increase the voltage of DC, it must be converted to AC in the process.
AC is not so simple: you cannot store AC in a battery, it must come from an AC generator (also called an "alternator" simple AC motors must have additional parts to make the motor start properly. however, for industrial applications, "three phase" or "rotary" power is applied. this, while complicating the wiring a little bit, makes the motor even simpler than a DC motor. AC can also be "transformed" to a different voltage, simply by using a magnetic transformer. This is the primary reason why most power distribution systems use AC power: they send it from the power plant at about 100,000 volts; at the substation, they transform it to around 10,000 volts, and at the pole outside your house, they reduce it to 240 volts (in America - voltages change from country to country) then they split that 240 volts into TWO circuits at 120 volts.