Fluid Dynamics is a branch of knowledge that deals with the motion of fluids. Dynamics, which is concerned with the study of forces and their effects on motion, is divided into two fields—dynamics of rigid bodies and dynamics of non-rigid bodies. The latter is usually further divided into two general classifications: elasticity (solid elastic bodies) and fluid mechanics.
Fluids are classified as liquids or gases, but in simple terms, a fluid is a substance which cannot resist a shear force or stress without moving, in contrast to a solid. Liquids have intermolecular forces which hold them together so that they possess volume but no definite shape. A liquid poured into a container will fill the container up to the volume of the liquid regardless of the container’s shape. Liquids are only slightly compressible and their density only varies slightly with temperature or pressure.
Gases, on the other hand, consists of molecules in motion which collide with each other tending to disperse them so that a gas has no set volume or shape. A gas will fill any container into which it is placed, and for a given mass, its volume, pressure, and temperature are related by an appropriate equation of state, such as the ideal gas law.