Rough Vacuum: Atm (1000mbar) - 10^-2mbar
Rough vacuum is concerned with the removal of the bulk gas from the system. There are many, many gas molecules in the chamber (right) and these interact with each other according to the laws of thermodynamics in the manner of a viscous fluid. The gases are said to be in 'viscous flow'. Rough vacuum pumps are therefore fluid flow pumps of the sort familiar to most mechanical engineers.

Process Vacuum: 10^-2mbar - 10^-4mbar
Many vacuum processes occur at a pressure of the order of millionths of an atmosphere. The chamber is first evacuated to high vacuum (see below) and then back-filled with some process gas. At these pressures interactions between molecules are still significant but the fluid flow characteristics of the gases breaks down and gas collisions with the chamber walls also begin to affect the gases' behaviour. Now, in addition to the gases flowing through the system, trace contaminant gases are desorbed from the chamber walls. Few pumps are optimised for process pressure operation and a combination of rough and high vacuum pumps (in series) is usually required to obtain the required conditions.

High Vacuum: 10^-5mbar - 10^-9mbar
The high vacuum regime is dominated by molecule - chamber wall collisions with the mean free path between molecule - molecule collisions being far greater than the dimensions of the chamber. The residual gases bounce around like marbles being shaken in a box and a completely different kind of pump is needed. Rather than literally sucking the gases out of the system, the pump must wait, like a Venus Flycatcher, for gas molecules to enter its throat. High vacuum pumps are therefore 'statistical capture' pumps. High vacuum pumps cannot pump atmospheric pressure gas and cannot be exhausted to atmosphere. Rather, they are secondary pumps and require either 'backing' or periodic 'regeneration' by a rough vacuum pump.

Ultra-High Vacuum: < 10^-9mbar
Whereas the dominant species in high vacuum is usually water, ultra-high vacuum (UHV) is almost 100% dry and Hydrogen is the most prevelant residual gas. Hydrogen is light and mobile and very difficult to pump, requiring specialised UHV pumps and reducing the gas load from the chamber walls is paramount.