Normal air pressure: 760 Torr pressure
Low Vacuum: 760 .. 25 Torr
Medium Vacuum: 25 .. 1e-3 Torr
High Vacuum: 1e-3 .. 1e-8 Torr
Ultra High Vacuum (UHV): 1e-8..1e-12 torr (Our base pressure is about 5e-10 Torr, but is usually higher since we do actually work in our system
)
Vacuum better than that is called "Extremely High Vacuum", but that requires a lot of tricks and is usually found only in a handful of specialized labs around the world. UHV is comparatively common -- most research universities have a few labs each reaching that.
To compare: A typical space shuttle low earth orbit is about 1e-6 Torr. Hubble is a bit better: ~1e-8. The moon's surface is 1e-11 Torr. That still means 40,000 molecules per cubic centimeter. Interplanetary space has 10 molecules per cubic cm, interstellar space 1.
All those molecules zipping around may stick to surfaces, say, of our sample, which we want to keep as clean as possible. One metric is how long it will take for one atomic layer of unwanted crud to stuck onto our surface. In normal air, this time is in picoseconds, essentially immediately. In 1e-6 Torr vacuum, it's a minute. In 1e-8 Torr an hour. 1e-9, about 10 hours, or a workday, so that's something we can work with.