Electrical systems differ around the world. Some use 50 hertz; some 60. Some use 110-120 volts; others 220-240. Some are on all the time, barring an ice storm or earthquake; some run a few hours a day. The plugs are different. The traveler, if he is bringing electrical appliances, must be prepared to adapt.
It can be daunting, but it actually isn't too hard. There are only two main type of electric systems used around the world, with varying physical connections:
If the voltage and frequency for your device is the same as where you are travelling, then physical plug may be the only thing that's different. Many of the electrical adapters for laptops, cell phones and can take either system, needing only a physical adapter. Read the label on your adapter for details. If it says something like:
INPUT: 100-240v, 50/60 hz
then this is the best situation, where you only need a physical adapter.
In general, countries use two or three prongs in different configurations. If you need to convert from one electrical system to another, that's called an step up or step down transformer, which is larger, heavier and actually has circuitry to step up or step down to the other system.
For example, between England and Germany, you need only an adapter. You stick your English plug in the adapter, which connects the rectangular + and - prongs to the round German ones and puts the ground where the German outlet expects it, and you're good to go. If you're an American traveling to Europe, you'll need a voltage converter, except for devices, such as computers, which have power supplies that can take either voltage. If the frequency is different, don't take a plug-in clock - it'll show 20 or 28.8 hours in a day.