What is a Vacuum Pump, what does it do?

Pumps can be broadly categorized into three techniques

Positive displacement pumps use a mechanism to repeatedly widen a cavity, allowing gases to flow through the chamber, closing the cavity and venting it to the atmosphere.
Momentum transfer pumps, also called molecular pumps, use high-speed jets of dense liquid or high-speed rotating blades to expel gas molecules from the chamber.
Entrapment pumps capture gases in a solid or adsorbed state. This includes cryopumps, receivers and ion pumps.

A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. Vacuum pumps are used in a variety of applications, including air conditioning, refrigeration, and medical equipment. Vacuum pumps can also be used to create a vacuum for processes such as degassing, distillation, and freeze drying.

Positive displacement pumps are most effective for low vacuums.
Momentum transfer pumps, along with one or two positive displacement pumps, are the most common configuration used to achieve high vacuums. In this configuration, the positive displacement pump serves two purposes.
Since momentum transfer pumps cannot start pumping at atmospheric pressures, the momentum transfer pump first achieves a rough vacuum in the evacuated vessel before being used to achieve high vacuum.
Second, the positive displacement pump backs up the momentum transfer pump by evacuating the accumulation of displaced molecules in the high vacuum pump to the low vacuum. Entrapment pumps can be added to achieve very high vacuums, but they require periodic regeneration of the surfaces that trap air molecules or ions.
Because of this requirement, usable run times at low and high vacuums can be unacceptably short, so their use is limited to ultra-high vacuums. Pumps also differ in details such as manufacturing tolerances, seal material, pressure, flow, acceptance or rejection of oil mist, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to fluids and vibration.