The term "roughing pump" derives from the vacuum range it works in, "rough vacuum", above 1x10−3 torr (0.1 Pa).
Pumps that operate in the high vacuum ranges typically don't operate, or only operate inefficiently, at atmospheric pressures, whereas pumps that work efficiently at atmospheric pressure usually cannot produce a vacuum lower than approximately 1x10−3 torr.
This concern of "hydrocarbon backstreaming" where pump oil as a gas makes its way into the vacuum chamber, has led to oil-free pump designs on the market.
Within the two categories are various kinds of designs with differences among them based on maintenance issues, initial costs, pump lifespan and vacuum level.
Early in their manufacture, dry pumps were significantly more expensive than oil-based pumps, however over time the cost gap has closed.