Corsair provides a thorough overview of power supply efficiency and of the 80 Plus program, if you'd like to learn more.
The great rail debate
In addition to identifying output power, manufacturers will specify the number of +12V rails their PSUs contain. A "single-rail" power supply has a single, high-power +12V rail for feeding power to hungry system components. A "multi-rail" unit divides its output between two or more +12V rails.
In a single-rail design, all of the power from the supply will be available to any component connected to the unit, regardless of the connector or cable used. In the event of a failure, however, a single-rail power supply has the potential to shoot much more current into your components.
Meanwhile, the main disadvantage of a multi-rail PSU is that it can't share power among the different rails. For example, if you connect 25 amps' worth of components to a +12V rail with a 20-amp maximum rating, the mismatch will trigger an overcurrent protection (OCP) mechanism and shut down, even though other rails my be available with plenty of power to spare. Consequently, with a multi-rail PSU you must pay attention to which components you've plugged in to which rail, a mild nuisance that you don't have to worry about with a single-rail power supply.
On the other hand, that disadvantage becomes a major advantage if you ever encounter a catastrophic failure. The OCP mechanisms in a multi-rail power supply monitor each rail and will shut the whole unit down if they detect an overload on any of the rails. The OCP on single-rail units kicks in only at much higher amperages, which could lead to a major melt-down if a serious overload occurs.