We commonly use the word vacuum to refer to any area that is of relatively low pressure. The air in areas of higher pressure will spontaneously move into areas of lower pressure, and this is the principle upon which a vacuum cleaner operates. A fan is used to push air out of the vacuum cleaner, creating an area of lower pressure behind the fan. Air from the outside then comes rushing in to reduce the gradient in pressure, carrying dust and dirt along with it. Since the fan is running continuously, the gradient in pressure is constantly being maintained, so the vacuum cleaner is able to keep running even though air is always flowing in.