^5 +1 what Viper said. You are well within operating specifications.
Different pc components have different voltage requirements. That's why you see a variety of voltages.
The +12 volt rail (or rails) supplies about 75% of the power required by a pc system. The rail supplies power to the cpu, the video cards, and other components. The cpu and the video card require the most power. That's why the +12 volt rail is often mentioned in a list of system requirements.