Electricity in Rotterdam
Rotterdam, like the rest of the Netherlands operates on a 220V/50Hz electrical system. The sockets in the wall require plugs with two narrow cylindrical prongs (also called type F) as you can see at the picture.
Use a converter if your equipment is not on 220V
You will need a converter if your electrical appliances are not 220V/50Hz, but most computers and charges currently are ready for both 110V and 220V use. It might be useful to check if your equipment can be used on 220V as well before plugging into the socket, because otherwise you might destroy your equipment!
Use an adapter to plug in your equipment
You might an adapter as well to be able to plug into the sockets. You can buy the adapter at almost any airport (including Rotterdam and Amsterdam airports) and in most electronics shops in Rotterdam. When you stay in a hotel you can check with the hotel reception if they've got adapters and/or converters on loan as well.