Receptacles (which most people refer to as outlets) come in many different styles, colors, shapes, and sizes. The typical house receptacle is rated 15-20 amp while most commercial receptacles are rated 20 amp. They can be for larger appliances and voltages, but lets just stick to the basics. You have all seen the 2 prong receptacles that were are used when there was no ground wire ran in the older wiring. While they can still be replaced with either another 2 prong or a 3 prong ground fault circuit interrupter most of the boxes you will find will be metal boxes. Something saying metal and no ground should bring attention to anyone replacing these receptacles. If metal is energized with no ground the metal will most likely  become energized as well setting up for a dangerous situation. Or at the very least a nice bright blue flash while trying to change the receptacle with the power still on!!

Leave it up to us to keep you from this situation. Often times they did not run pigtails to the older receptacles and attached either to the back or to the side of the receptacles. This causes another problem such as maybe you have a melted receptacle and half of your receptacles stopped working.