What is AC? What is DC? Describe the voltage in the wall outlet? Describe the voltage in USB 1.x and 2.x cables?

DC (direct current) flows in one direction at a (nominally) constant voltage—what batteries and most electronic supplies provide. AC (alternating current) periodically reverses direction and varies sinusoidally; it is used for power distribution because it is easy to transform between voltage levels.

Wall outlet: In North America it is approximately 120 V AC RMS at 60 Hz (the peak is about 170 V); in much of Europe and many other regions it is about 230 V AC at 50 Hz. The value quoted is the RMS voltage. (Older references may cite 110/115 V or 220/240 V for the same nominal systems.)

USB 1.x / 2.x: USB carries 5 V DC on the bus power (V_BUS) pins. A standard USB 2.0 port nominally supplies up to ~500 mA (USB 1.x ~100–500 mA depending on enumeration), with data on the separate D+/D− differential pair.