Crutschow certainly defined it for you. Can't say much more, but I will add that there are basically two useful forms of electricity, alternating current and direct current. alternating current can't be stored as in a battery, but it is more easily and efficiently transmitted from point A to B. DC can be stored, but transmission is more difficult over distances without heavy loss.
AC is easily converted to DC, but converting DC to AC (at higher power levels) is a little more complicated.
Because AC is delivered to our homes, but many of our useful devices require DC, we use DC power supplies to rectify AC and power our devices. A DC power supply comes in two forms, a circuit that rectifies AC into DC, or a battery of stored DC that was rectified and charged beforehand.
Hope this makes it a little clearer.
Oh, one thing to add. AC is NOT limited to commercial and residential power but comes in many forms and applications.