Surfaces are intrinsically less energetically favourable than the bulk of a material; if this were not the case then there would be a driving force for surfaces to be created, and surface is all there would be.
Surface energy arises as a consequence of the bond disruption which occurs when a surface is created. (see also adsorption) Cutting a piece of stuff in half, disrupting its bonds, will consume energy. If the cutting is done reversibly (see reversible), then the energy consumed by the cutting process will, by conservation of energy, be equal to the energy inherent in the two new surfaces created. Thus the surface energy of a material is equal to half of its energy of cohesion, all other things being equal. In practice, only a freshly prepared surface in vacuum meets this criterion. Surfaces are often found to be highly dynamic regions, which readily rearrange or react, changing their form away from the simple "cleaved bond" model described above.
As first described by Young in 1805 in the Philosophical Transactions of the Royal Society of London, it is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not wetting, the spreading of a liquid over a surface, occurs. If wetting does not occur, then a bead of liquid will form, with a contact angle which is a function of the surface energies of the system.