A solenoid is a 3 dimensional structure of wire. When this wire is wrapped around a metallic block in a coil and electricity is passed through it, it has some special magnetic properties. It turns into an electromagnet that can be switched on or off. The side in which the current appears to be passing clockwise, is the South Pole, and the side in which the current seems to be passing anticlockwise is the Northern Pole. The solenoid works just like a bar magnet, and therefore has many uses.
This principle is used to create valves, among other things.