Take the usual battery for example. It’s a device that has a load of chemical energy and it transforms it to electrical energy. They come in all sorts of sizes and shapes, from miniature ones used for hearing aids to car batteries.
One of the most used power supply is the switched mode power supply. Mostly we use this kind of supply for desktop computers. The AC voltage is filtered thus obtaining AC electrical flow, in order for your computer to function properly! The switching takes place at very high frequencies , usually form 10 khz to 1Mhz , therefore facilitating the use of transformers and capacitors that are cheaper, smaller and lighter than the usual linear ones, more information http://www.electroschematics.
One of the “smartest” power supplies are the programmable ones. In my opinion these are a must in every household that intensively uses hi-tech appliances such a as computers , home theatre’s etc., because you can set a certain value for the voltage, that the power supply recognizes and stops every piece of technology it is connected to, before it’s to late! This happens when abnormal high value voltage flow trough the electrical network and burns everything it encounters without any warning , even if all your stuff is turned off at the time.
There are other overload protections such as fuses and circuit breakers. The fuse contains a very small wire that will melt if to much electrical energy will flow trough! The breaker contains a trigger that shuts the circuit down , and it’s much safer if you ask me!