Galvanic isolation between AC input and DC output?
The input power (North America) is line and neutral. On the same connector is a safety earth connection.

The power supply galvanically isolates the input power from the output power (several voltages with a common). 

The earth prong is connected to the metal chassis for safety reasons (on desktop computers), including an internal connection to the metal case of the power supply itself. 

Typically, on virtually all desktop PCs and many notebooks, the output power common is also tied to the chassis and the earth prong. On some notebook computers, the chassis is floating.