The 12V rails on a power supply are marked by a yellow wire. 12V (DC) has developed over the years into the most important supply voltage within a computer and delivers power primarily to the graphic card(s), processor and (optical) drives.
20/24 pin (ATX connector)
The 20/24 pin connector – also known as the ATX plug – is the biggest and also the most important plug for a power supply unit. It is connected to the mainboard and delivers not only the three primary power voltages (3.3V, 5V and 12V) but also the power-good signal required for operation as well as 5V stand-by and -12V. The 20/24 pin connector is thus a computer's primary power supply connection.
4+4 pin (connector)
The 4+4 pin plug is also known as the 8-pin EPS connector. It contains eight cables, including four connected to the 12V rail. The other four cables connect the power supply to the earth. This plug delivers power to modern multi-core processors. The 4+4 pin is also informally known as the 8 pin connector or p8 plug.