Start by taking a look at the back of the device you want to use. If it says something like “100-240V, 50/60 Hz”, it will work anywhere in the world with the right adapters. Connecting a device to a lower voltage than it was designed for is generally not dangerous; the device may not work correctly, but no dramatic failure is likely. Giving any device a voltage higher than it was designed for is dangerous eg if you put 230 volts into a 110V it will melt or explode!
There are only two main types of electric systems used around the world:
100-127 volt, at 60 hertz frequency (in general: North and Central Americas, Western Japan)
220-240 volt, at 50 hertz frequency (in general: the rest of the world, with some exceptions)
Originally Europe was 120 V too, just like Japan and the US today. It was deemed necessary to increase voltage to get more power with less losses and voltage drop from the same copper wire diameter. At the time the US also wanted to change but because of the cost involved to replace all electric appliances, they decided not to. At the time (50s-60s) the average US household already had a fridge, a washing-machine, etc., but Europe did not.
But don’t despair, most mobile phones and digital cameras chargers work with both 110V and 240V systems, though you may still need an adaptor plug or have to use the shaver socket.
Here is a list of plug sockets and voltage for your next trip – http://www.kropla.com/electric2.htm