Loads are generally classified as "live loads" such as the weight of occupants and furniture in a building, the forces of wind or weights of water, and the forces due to an earthquake, or "dead loads" such as the weight of the building itself.
Structural engineers need to design structures so that while serving their useful function, they do not collapse, and do not bend, twist, or vibrate in undesireable ways. In addition they are responsible for making efficient use of funds and materials to achieve these structural goals.
One of the most straightforward mechanisms of analyzing structures is the method of Statics in which Newton's laws of motion are used to determine the forces acting on the components of a structure generally by assuming that the material is rigid and uniform.
Another mechanism capable of handling more complicated situations is the Finite element method which is capable of calculating forces in structures made of various materials with differing properties.