3v, 5v, 12v

[coolgod]

can anyone explain to me these 3v, 5v and 12v railing mean in a psu.
I've always thought the 12v is only in the 20-24 atx plug ans the 5-3v are in those 4 pin molex and floppy power, but after carefully looking at a case fan i found out those 4 pin molex can also support 12v? so what exactly is the 5v for? which line is which?

 

[gagaga]

12v is old school, but is used to deliver the serious power used by CPUs, 3.5" Hard disks, Optical drives, Graphics cards etc

5v is used by the rest of the motherboard, USB, laptop components (HDD and optical) etc

3.3v is where it's heading - newer components (parts of the PCIe bus??) plus it is part of the SATA power spec (1.8" SATA drives run on 3.3v)

MOLEX is 12+5+0v. SATA power cables are 12+5+3.3+0 (yellow+red+orange+black)