That part is known as either a wind deflector or a Roof Fairing. I believe the latter is the correct technical term.
To satisfy your curiosity about the other parts as well. I know it is a little more than you asked but what the hell I'm on a roll.
1: Tractor Unit (or Truck)
2: Semi- Trailer.
10: Pedestal (crank unit has 2 speeds)
11: Trailer Dolly( which can slide backwards or forwards and be locked in position). This is done to increase or decrease weight capacity of the payload and also to make backing up to the dock easier by shortening turning radius. Weight is measured on weigh scales between # 8 and #11
8: Tractor side: Fifth Wheel, Trailer side: King Pin