A vacuum chamber is a hard enclosure from which all the gases and air are discharge thanks to a vacuum pump.
Vacuum chambers are required to avoid contamination of samples in many disparate kinds of spectroscopy and for thin film depositions. Most vacuum chambers are fabricated in aluminum because it is nonmagnetic and so it allow to controll magnetic fields from outside the chamber.
Aluminum is a good choice also because compared to other types of metals, it absorbs minor water and trace gases.
UHV (Ultra high vacuum) means having pressures lower than about 10-13 atmosphere.
Vacuum chambers can be really useful: even at 10-10 atmosphere, it takes only 1 second to cover a surface with a contaminant.
For more info about it click here and read the wikipedia complete description.