See, not every country in the world uses the same shape plugs. I've heard of about thirteen distinct sizes and shapes of electrical plug. Worse still, some countries have outlets that put out about twice as much power as others. So even if you managed to force the wrong shape plug into the socket, that hair dryer might just explode. And that's no fun.
So, what's a traveler to do? The answer has two parts: plug adapters and voltage converters.
Read this next line carefully and remember it: you might not need either of them, and even if you need one, you might not need the other.
A lot of people make the mistake of bringing too many gadgets to make their devices work abroad. It doesn't hurt to be cautious, but some of these things just weigh you down. The most common mistake is to bring a voltage converter when you don't need it. But I'm getting ahead of things-- first I'll explain what these things do.
An adapter is the thing you use to make the plugs from home fit in the sockets of the country you're traveling through. So if you have a flat, two pinned US plug, you stick it in a little adapter with a round, two pinned European plug sticking out. Voila, European power to your American device.
The converter is an extra box that goes between your cord and the outlet to make sure you're getting the right amount of European power to that American device. European power outlets for example, give out twice the voltage of their American brethren. So something designed to handle half the voltage could be fried. The converters take care of it by either cutting the voltage down or raising it up depending on a little switch on the back.
Most people get this far on their own. Here's the stuff not everybody gets on adapters and converters:
First, with adapters, yes, there are tons and tons of different kind of plugs you can get adapters for, but no matter where you go in the world, you only need three. There are four basic kinds, and your devices almost certainly already have one of the four, leaving you with three you'll need adapters for. While the outlets might be different in thirteen different ways, one of these four plugs will almost always work. They go like this, as pictured above from right to left: Two thin, flat, parallel prongs (North America, most of South America and Asia), Three chunky rectangular prongs (the UK and most of its former colonies in Africa), two round prongs (continental Europe, a small chunk of South America, most of the Middle East and northern Africa), and thin, flat prongs at diagonal angles to each other (Australia and New Zealand). I've traveled with these four in all of these places, and while I occasionally find an outlet that I can't plug into, I can usually find one in the same room that I can. South Africa, Namibia, and India, for example, all use three round prong plugs, but so many devices are sold in these countries with European plugs, that almost everyone has at least one euro adapter lying around. So even if you don't see it in the wall, a euro plug will get you by just fine.
Second, converters. If you're packing only electronic devices (cell phones, iPods, laptops), you probably don't need a converter. If you're packing more basic electrical plug-in stuff like a hair dryer, you probably will need one. Look at the power cord for an A/C adapter. It'll be a chunky thing at the end or in the middle of the cord somewhere with a sticker or writing etched into the side. Here are the magic words you're looking for: "Input: 100-240V" If it says this or something a lot like it somewhere, it's designed for travel and doesn't need a converter. If there's nothing like that on the plug anywhere (i.e. it's just a cord with a plain plug on the end) it will need a converter.
In my opinion, if you need a converter to use it, you shouldn't bother packing it. Converters are big, heavy, and cost a lot more than a simple adapter set. I don't carry anything that needs a travel converter.
Finally, if you've got a little bit of extra cash, consider going solar. If you look online, you can find compact solar arrays for basic electronic devices like cell phones and iPods. I have one for rechargeable AA and AAA batteries (though it does also plug into the wall). I've met other travelers who have the ones for other devices, and they have been quite pleased with them. Obviously they're only really good for charging batteries where you have some sunlight or fluorescent bulbs, but sunlight is often a lot easier to find than a free electrical plug.
Enjoy, and be thankful I'm not ending this with a stupid pun on the word 'power.'