80% efficiency for bronze and like 2.5% more efficient per up.
Do not worry about it too much-focus on brand and price.

These are efficiency certifications. More efficiency can equal lower electricity bills, and less energy is wasted within the power supply resulting in lower temperatures therefore lower fan noise if your PSU's fan is controlled based on PSU temperatures (most are). Requirements for each certification are as follows.

80 Plus

20% Load: 80% Efficient
50% Load: 80% Efficient
100% Load: 80% Efficient

80 Plus BRONZE

20% Load: 82% Efficient
50% Load: 85% Efficient
100% Load: 82% Efficient

80 Plus SILVER

20% Load: 85% Efficient
50% Load: 88% Efficient
100% Load: 85% Efficient

80 Plus GOLD

20% Load: 87% Efficient
50% Load: 90% Efficient
100% Load: 87% Efficient

80 Plus PLATINUM

20% Load: 90% Efficient
50% Load: 92% Efficient
100% Load: 89% Efficient

The standards for 80 Plus PLATINUM have actually been lowered from 90%/94%/91% at 20%/50%/100% load respectively. But efficiency is NOT the most important factor about a power supply. You also want something with high end internal components such as good capacitors - Japanese brands are preferred especially Nippon Chemi-Con/Rubycon/Nichicon.

Today's most preferred power supplies have independent voltage regulators and a DC-DC converter for the minor rails. You want voltage regulation to be within ATX specification (5% deviation). Too high voltages result in BSODs or damaged hardware. Too low results in automatic shut downs or reboots of the PC. You also want ripple suppression to be in specification - 120 mV on the +12v rail and 50 mV on the +3.3v/+5v rails. Ripple voltages are the waveforms found in AC current. Too much of it can damage hardware.

If you're looking at a 1000W power supply, right now one of the best ones you can get is the Silverstone Strider Plus 1000W. It is 80 Plus SILVER certified, fully modular, relatively short in length (180mm), and it is a great performer. Much better than the Corsair HX1000 which costs more. Independently regulated, good +12v ripple suppression. +12v performance matters the most since this rail powers modern motherboards, CPU cores, video cards, drives and fans.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817256057&cm_re=st1000-_-17-256-057-_-Product

Here is a review on it.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=180

Basically the higher the 80+ certification the more efficient which means less power consumption and heat output.

thanks everyone for giving me great answers :D