ACPI defines several power states. Depending on the support of your motherboard, BIOS and OS, some of these may not be available to you:
G2: Soft off. The OS is shut down and the system is powered down. Most modern desktop PCs enter G2 when the user tells them to "Shut Down". Some events can cause the system to restart; the most common event is the user pressing the Power button.
G3: Mechanical off. The OS is shut down and the system is disconnected from the power supply (usually by a switch on the back of the PSU). Upon reconnection, the system enters G2 without further intervention.