80 PLUS is an initiative to promote energy efficiency in computer power supply units (PSU). It certifies products that have more than 80% energy efficiency at 20%, 50% and 100% of rated load, and a power factor of 0.9 or greater at 100% load. That is, PSUs that waste 20% or less electric energy as heat at the specified load levels, thus reducing electricity use and bills compared to less efficient PSUs. Sometimes rebates are given for manufacturers who use 80 PLUS-certified PSUs.
There's 5 levels:
Obviously, Platinum rated supplies will be the most efficient of the bunch.
There is a nice table outlining the efficiency percentages required for each rating here:http://en.wikipedia.org/wiki/80_PLUS
beat me to it