To those who asked about our Christmas tree yesterday, there are two things you have to keep in mind when comparing North American and European power systems. There are two basic standards for voltage and frequency in the world. One is the North American standard of 110-120 volts at 60 Hz, and the other is the European standard of 220-240 volts at 50 Hz. There are also different plugs, that have different shapes.
Some electronic equipment, such as computers, camcorders, and other "high tech" equipment, have a built-in transformer, that will transform the voltage and frequency from one to another, so you can use North American equipment in Europe, and vice versa, as long as you have a plug adapter. Other pieces of electronic equipment (such as lamps, treadmills, most small kitchen appliances, etc.) do not have built-in transformers, and so if you want to use a North American appliance in Europe, or vice-versa, you not only need the plug adapter, you also need a transformer. Transformers can be tricky things to use at times, because you need to ensure that the polarity is correct, otherwise you can destroy your equipment or appliance.
So my humans can use their North American laptop and camcorder here in Germany, with only a plug adapter, since those two pieces of electronic equipment have a built-in transformer. All of the other electrical appliances in the house (all the lamps, the TV, treadmill, microwave, electric kettle, toaster, etc.) were bought in Germany, since they don't have built-in transformers, and so just using a plug adapter would damage or destroy the appliance.