Engineering science is usually subdivided into number of topics such as
1. Solid Mechanics
2. Fluid Mechanics 
3. Heat Transfer 
4. Properties of materials and soon Although there are close links between them in terms of the physical principles involved and methods of analysis employed.

The solid mechanics as a subject may be defined as a branch of applied mechanics that deals with behaviors of solid bodies subjected to various types of loading. This is usually subdivided into further two streams
1. Mechanics of rigid bodies or simply Mechanics 
2. Mechanics of deformable solids. 
The mechanics of deformable solids which is branch of applied mechanics is known by several names i.e. Strength of Materials, Mechanics of Materials etc.

1. Mechanics of rigid bodies: 
The mechanics of rigid bodies is primarily concerned with the static and dynamic behaviour under external forces of engineering components and systems which are treated as infinitely strong and undeformable Primarily we deal here with the forces and motions associated with particles and rigid bodies. 
2. Mechanics of deformable solids (Mechanics of solids):
The mechanics of deformable solids is more concerned with the internal forces and associated changes in the geometry of the components involved. Of particular importance are the properties of the materials used, the strength of which will determine whether the components fail by breaking in service, and the stiffness of which will determine whether the amount of deformation they suffer is acceptable. Therefore, the subject of mechanics of materials or strength of materials is central to the whole activity of engineering design. Usually the objectives in analysis here will be the determination of the stresses, strains, and deflections produced by loads. Theoretical analyses and experimental results have an equal roles in this field.

Conclude :
Strength of material is the study of the effect of forces in causing changes in the size and shape of the bodies by dealing with their strength, stability and rigidity. 
It includes the study of elastic properties of materials and their behavior during interaction with external forces. 
The particles of rigid body are connected together by force of cohesion, and this force must be completely overcome before the body can be ruptured. 
Force acting upon a rigid body, first tends to change the relative position of its particles, and finally perpetrates them from each other. 
Such forces are generally applied to bodies slowly, and the changes in size and shape occur while the forces are increasing up to their final value. 
Elasticity is the property which bodies possess by taking new form under the action of force and also of getting their original form after removing of force.