Fluid is a substance which includes both liquid and gas. It regularly deforms if shear is applied on it. It is considered a composition of molecules in microscopic or both microscopic and discrete states. The features which distinguish a fluid from a solid are the lack of ability of a fluid to resist shearing stress. The fluid experiences random deformation under the action of forces is manifested in the tendency of fluids to flow. On contrary, fluid deforms endlessly when subjected to a shear stress, no matter how much small that stress may be. A finite number of particles size within fluid is much larger than a molecule however infinitesimal in size compared to the total volume of the fluid. Internal resistance is experienced by the deformation of fluids which is introduced for the help of shearing stress. The deformation is going on until the applied shear and the shear resistances are not equal. Fluid properties depend on the molecular structure and the nature of forces between the molecules.