All of what has been said is very true. Just adding a few things. DC motors come in a lot of different voltages. As has been said before you just cant slap some leads on a DC motor and plug it in. You might have a DC motor that runs on 4-5 volts lets say which is a typcial voltage in a digital board. That means that you have to take the 120 v AC from the wall and first convert it to a DC supply (rectify it as someone has said... this can be done by several different ways....wont go through that here) then bring the voltage all the way down to the needed 4-5 volts (usually by a transformer). Also remember that an AC voltage is a sine wave. This means that it goes up and down from some positive voltage to some negative voltage. It follows this path at a frequency..... if its from the wall this is 60 Hz (60 cycles per second... that is the voltage goes from the some positive value, passes zero, then goes to the some negative value 60 times a second).
Hope this helps a bit.
Michael
