The +3.3V line is used was used by the processor, motherboard, some fans, and every DELs connected to the power supply. Nowadays it is being used only by LEDs, and on some motherboard it was abandonned altogether.
The +5/+12V powers everything else: HDD needs both, graphic cards need both, CPU and RAM needs +5V or +12V depending on the motherboard.
But if I remember right, Intel decided about 2 years ago that the industry have to abandon +3.3V and +5V, and use only +12V, even some power supplies have worked on this and provide only +12V. One day every manufacturer will move to +12V. On some motherboard the cpu has been moved from +5V to +12V, the same goes for RAM, but harddisk/optical drives still uses both +5.5V AND +12V at the same time. The same for LEDs and some fans. The latest fans on the market uses +12V only and modifies it so they can get 3.3, 5, 7, 9 and 12V. Also as some members told you, PCI-E 6/8 pins connectors uses +12V only.
Hope it answers your question. It all depends on your current hardware. If you want to be absolutely sure which one is use by which hardware, provide us with your exact hardware, or you can look inside the hardware manufacturers manuals/website to get that info.