In chemistry, adsorption of a substance is its concentration on a particular surface: the molecules of a liquid or gas form a film on a solid body.
Consider a clean surface exposed to a gaseous atmosphere. In the bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are filled. However, by definition the surface represents a disruption of these bonds. It is then energetically favourable for these dangling bonds to react with whatever happens to be available. The exact nature of the bonding depends on the details of the species involved, but the adsorbed material is generally classed as being either physisorbed or chemisorbed.
The simplest form of adsorption, physisorption, is due to weak forces of attraction, typically Van der Waals forces. As these forces are ubiquitous, it follows that any clean exposed surface will rapidly accumulate a layer of physisorbed material.
Chemisorption occurs when a chemical bond, defined in this case as an exchange of electrons, is formed. The degree of exchange, and how symmetric the exchange is, depends on the materials involved. There is often a close parallel with the situation encountered in coordination chemistry. Chemisorption is particularly important in heterogeneous catalysis, the most commonly encountered kind in industry, where a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, alterating the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.
The amount of material which accumulates depends on the dynamic equilibrium which is achieved between the rate at which material adsorbs to the surface and the rate at which it evaporates. The higher the rate of adsorption and the lower the rate of desorption, the greater the fraction of the available surface which will be covered by adsorbed material at equilibrium.