turbomolecular pump — Momentum transfer pumps, are also called molecular pumps, and use high speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber. The two main types of molecular pumps are the diffusion pump and the turbomolecular pump.
A turbomolecular pump is a type of vacuum pump, superficially similar to a turbopump, used to obtain and maintain high vacuum. These pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface. In a turbopump, a rapidly spinning turbine rotor 'hits' gas molecules from the inlet of the pump towards the exhaust in order to create or maintain a vacuum.
References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/wiki/Turbomolecular_pump