Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate.  New chemical bonds are generated at the adsorbent surface.  Examples include macroscopic phenomena that can be very obvious, like corrosion, and subtler effects associated with heterogeneous catalysis, where the catalyst and reactants are in different phases. The strong interaction between the adsorbate and the substrate surface creates new types of electronic bonds.
In contrast with chemisorption is physisorption, which leaves the chemical species of the adsorbate and surface intact. It is conventionally accepted that the energetic threshold separating the binding energy of "physisorption" from that of "chemisorption" is about 0.5 eV per adsorbed species.
Due to specificity, the nature of chemisorption can greatly differ, depending on the chemical identity and the surface structural properties.
The bond between the adsorbate and adsorbent in chemisorption is either ionic or covalent.