I'll give a brief overview. There are 3 different types of PSUs. Junk ones that can't output what they claim like the Diablotek. It might say 1050W, but it can barely do 500. Then there are quality units that can output the full amount, but are less expensive then the last type. These are typically the Corsairs and Antecs. (both companies have some models in the last category, but for the most part they fall into the second type. Then finally you have the third type, those that are quality and flaunt it. Enermax or PCP&C are often such examples.
Assuming you are asking about the difference between type one and two, one lies about how much power they can output, and uses an older design that is less efficient then the newer ones. This means they need to pull more power out of the wall in order to deliver the power the computer needs. Not only does this cost you more money, but it tends to create more heat in the computer, and because of the fans your room.