The output voltage of a power supply is usually multiple. There will be several different voltages listed on the back of the power supply and these correspond to the output of the various connectors that branch off of the power supply.
These connectors determine which components that they can be hooked up to. It’s nearly impossible to hook up the wrong power supply to a component in a computer. Most power supplies come with enough outputs—and adapters can be used, as well—to power the majority of motherboard. For builders, however, it’s imperative to have the right voltages and numbers of connectors on the power supply they use for their build.