A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container. A humectant is also a hygroscopic substance, but it is used to help a product retain water. Humectants are frequently used in food and cosmetic products. Glycerine is often used as a humectant.
Commonly encountered pre-packaged desiccants are solids, and work through absorption or adsorption of water, or a combination of the two. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules.
Pre-packaged desiccant is most commonly used to remove excessive humidity that would normally degrade or even destroy products sensitive to moisture. Some commonly used desiccants are: silica gel, activated charcoal, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves.
A specific usage of a desiccant would be in insulated windows where it is placed inside the spacer between the air space and window edge. This prevents moisture condensation between the panes.
Read more about Desiccant: Applications, Toxicity, Chemistry, Performance Efficiency, Coloured Saturation Indicators, Drying of Solvents, Types of Desiccants