Modern power supplies focus on providing +12 volt rails.
The power supply provides electricity to the motherboard and its onboard components through two different primary power connectors. The power supply also provides electricity to other devices installed to the system, such as the hard disk and the graphics card. The ATX form factor -- the primary form factor designed for desktop computers -- requires different types of power connectors and voltage lines as the needs of computers change.
P1 Connector
The power supply's primary power cable, the P1 connector, supplies +3.3, +5, -5, +12 and -12 volts to the motherboard. The connector dedicates an independent +5 volt rail ("+5 V Standby" or "5SVB") to the supply standby power to the motherboard while the power supply is inactive. The 5SVB enables the computer to wake from sleep mode.
P1 connectors originally were 20 pins in size, but they expanded to 24 pins to supply additional power to PCI Express slots. P1 connectors are backward compatible, so you can plug a 24-pin power cable into a 20-pin slot, and a 20-pin power cable into a 24-pin slot.
P4 Connector
The P4 connector, or ATX12V four-pin power cable, provides two additional 12-volt rails to the motherboard. The P4 connector chiefly supplies power to the CPU (Central Processing Unit), or processor, although it occasionally supplies power to expansion cards as well. If a P4 interface connector resides on the motherboard, the processor will fail to receive power unless you insert the ATX12V four-pin power cable into the connector. High-end motherboards and processors receive power through an extra eight-pin EPS +12 volt power cable ("EPS12V"). One end of the EPS12V can fit into a P4 connector.
Device Cables
The power supply not only provides power to the motherboard and its onboard components, but to other devices installed to the computer as well. The hard disk, optical drive, graphics adapter and floppy disk drive all receive power from the power supply. SATA (Serial ATA) hard disk and optical drives receive power via a SATA power cable, whereas PATA (Parallel ATA) devices require a four-pin peripheral power cable (also referred to as a "Molex" cable). Floppy drives require their own four-pin floppy drive cable.
High-end graphics cards (as well as some other high-powered PCI Express expansion cards) require either six-pin or eight-pin PCI Express power cables.
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