I'm living in the Netherlands. I'm going on a holiday to Thailand. I need to know which electrical plug is used over there?
A European one? or like the one in the States? or something else?
Contrary to Mark's post, you will likely not need an adapter coming from the Netherlands.
Thailand uses both the round pin plugs common in Europe as well as the flat pin plugs common in the USA & Canada. The wall outlets here accommodate both style plugs, using sockets similar to the one shown in pnuts comment link - http://www.thailandguru.com/electricity-220volts-thailand.html
However, grounded sockets like the one shown on the thailandguru page are not common, a fair majority of outlets are ungrounded. If your devices use a grounded plug, then you may need a three pin to two pin cheater plug. But otherwise your devices will plug in just fine.
Or for a picture version (albeit comparing them to Australian plugs, but at least it shows you) - this will show you what they look like.
Basically, international outlet/plug types A and C, and they run at 220V/50Hz.
Coming from the Netherlands, you will need an adaptor.