The power supply in the Netherlands is 230 volts. To be more specific: 220-240 volts AC (Alternating Current), 50-60 hertz. The same voltage is used over the rest of Europe.
Originally Europe was 120 volts, just like Japan and the U.S. today. But it was considered necessary to increase the voltage level in order to get more power with less loss of current and voltage drop when using the same diameter of copper wire. At the time the U.S. also wanted to change, but because of the costs involved to replace all the electrical appliances, they decided not to go ahead. At the time (1950s-60s) the average U.S. households already had fridges, washing-machines, etc, but this was not the case in Europe.
Most electrical devices you may want to bring (e.g. hairdryer, battery charger) will usually switch automatically to 220 volts, or can be switched manually. If in doubt, consult the manual. If your equipment does not run on 220 volts, then you should buy a 110>220 volt converter suitable for small appliances up to 1500 watts. Do not try to plug your non-switchable 110 volt appliances directly into a European socket without a transformer - you might destroy your appliance and possibly even start a fire. In some hotels you often find bathroom shaver outlets with both 110 and 220 volts available.
Electricity plug shapes, plug holes, plug sizes and sockets are different in many countries. In the Netherlands the standard continental 2-pin CEE 7-17 plug and CEE 7 plugs and sockets are used (check the images). If necessary you should bring a plug adapter with you – we advise you to buy one in your home country.