Dehumidifiers remove liquid from the air around them. Most either use electricity to cool the air, condensing water out of it, or some kind of dessicant (highly absorptive substances like silica gel) to absorb moisture passively. Some models, like ionic membrane dehumidifiers, remove water from the air while keeping it in in vapor form.
There are all sorts of reasons to dehumidify a space—from respiratory health to mold and mildew prevention to odor reduction. Dehumidifiers are common in commercial buildings with major water elements, like swimming pools and ice rinks, or sensitive materials, like storage warehouses and manufacturing plants.
Applying drinking filters to captured gray water can turn dehumidifiers into almost magical sources of water for safe use in cooking and cleaning use. Unfiltered dehumidified water can be used for cleaning purposes, or to water plants. Electric models tend to have a slight warming effect on spaces in which they're used, since the energy expended to cool air enough for condensation remains in the local environment, warming the surroundings.
American inventor Willis Carrier is credited with creating the first dehumidifier in 1902, to improve production conditions in a printing factory in Brooklyn.