Reliability:

Despite their low cost, (about 5-10% for a quality PSU) when compared to rest of the system components, power and cooling are responsible for about 30% of a systems problems. As you can see, by trying to save money on the power supply is not a good choice; especially since a good quality power supply and computer encolsure will last through several computer upgrades. It is not recommended to purchase a case and assume that a power supply is fine due to the high wattage rating.

Power Factor:

The power factor refers to the ratio of the actual power used by the device in relation to the production of the current and voltage supplied.  For an average power supply, this may be 60-70 percent.

Voltage Ratings Explained:

-12V: This voltage is used on some serial port circuits; especially on the ones whose amplifier circuits
require both -12V and +12V. Please keep in mind that this power rating is not needed  on some newer systems.  Even older systems did not require this power rating due to the less power consumption by the serial ports. Most power supplies  provide it for compatibility with older hardware, but usually with a current limit of less  than 1A.
 
-5V: -5 V was used on some of the earliest PCs for floppy controllers and other circuits used by ISA bus cards. It is usually provided, in small quantity (generally less than 1A), for compatibility with older hardware.
 
0V: Zero volts is the ground of the personal computer's electrical system, also known as "common".
The ground signals provided by the power supply are used to complete circuits with the other voltages. They provide a reference point against other voltages that are measured.

+3.3V: This is the newest voltage level provided by modern power supplies, it  was introduced with the ATX form factor and in most cases sends the power to the microprocessor.

+5V: This voltage is used to run the motherboard, the processor (directly or indirectly) and the vast majority of other components in the system. On newer systems, many of the components, especially  the CPU, have migrated to the lower +3.3V described above, but the motherboard and many of its
components still use +5V.
 
+12V: This voltage is used primarily to power hard disk drive motors. It is also used by fans and  other types of cooling devices. It is, in most cases, not used by the motherboard in a modern PC, but is passed on to the system bus slots for any cards that might need it. Of course, drives are connected directly to the power supply through their own connectors.

You will sometimes see the different voltage levels of the power supply refered to as rails. This definition comes from the fact that inside the power supply there is a strip or bar that provides each voltage level. 

When the computer power button is first pushed and the power supply starts up, it takes some time for the components to start producing the voltages that the computer needs to operate. Before this time, if the computer were allowed  to boot up, all kinds of strange occurances would happen since the power would not be at the right voltage. It takes a good portion of a second for the power to stabilize. To prevent the computer from booting prematurely, the power supply sends out a signal to the motherboard, which is referred to as - Power OK - after it completes its internal tests and the power is ready for use. Until this signal is recieved, the motherboard will not start up the computer.

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