The function of the power supply is to transform the 115V or 230V AC power into 3,3V, 5V and 12V DC power for the motherboard and peripheral busses. Voltages lower than 3,3V , e.g. for the most modern CPUs, are generated by a Voltage Regulator Module on the motherboard. Modern power supplies can be used on various voltages (110, 115 and 230V) and are also called switched-mode power supplies (SMPS).
In notebooks, the power supply is almost always external and is probably the only feature where energy efficiency is not an issue, because it doesn't influence battery life.
Power supplies for desktop PCs have typical rated efficiencies of 65-75%. For special (more expensive), 'silent' PCs efficiencies are in 80-90% range. Currently, ENERGY STAR qualified computers have an internal power supplies must have a minimum efficiency of 82%.
Efficiencies of external power supplies start from 65%. Nowadays most of them have on their nameplate an indication of their efficiency: the International Efficiency Mark. This is a roman numeral (e.g. II, III, IV). At this moment a mark V represents the most energy efficient type of external power supply, which is also required for ENERGY STAR qualification.
Since June 2000 there is a Code of Conduct between external power supply manufacturers at the initiative of the European Commission which should improve things, at least as far as no-load power consumption is concerned.
Courtesy of the French Energy Agency ADEME, Future Electronics project. Expanded by VHK.