In Ansys workbench , structural module, in modelling, what is the exact difference between pressure and surface force application? Is surface force applied in same manner as that of pressure application or different? In more detail, I want to know how the total force is applied on the surface equally?
"Pressure" is always normal to the surface. "Surface force" can be applied in any direction.
As example of a surface force that is not normal to the surface would be snow loading on a sloping roof - the weight of the snow acts vertically downwards, not perpendicular to the slope of the roof. (Of course there must be some friction between the snow and the roof, otherwise the snow would just slide down the roof.)
If you are doing a nonlinear analysis, a pressure load can change direction as the structure deforms, to stay normal to the surface. A surface force (like the snow loading) may always act in a fixed direction.