Adhesion is surface related. Any process depending on surfaces to stick to each other is an adhesion process: soldering, brazing, adhesive bonding, and coatings. Some things join or seal two surfaces. Others protect the surface or improve its performance for wear, thermal resistance, corrosion, or oxidation.
It is possible to join dissimilar materials using adhesion processes. It is desirable to use those methods when damage to the base material might occur from overheating or ther nastiness. It is also possible to change surface characteristics and performance by applying an adherent layer of a dissimlar material, like putting rubber onto a surface to protect it from abrasion, or putting ceramic on a surface also to protect it from abrasion. Get it?
Call me if you want to discuss an adhesion issue or troubleshoot something that is supposed to stick to something else.