We know, there are two types of matter. One is solid and the other one is fluid. Fluids are classified into two groups: viz— liquid and gas. Any fluid travels from one place to another. By throwing water on the floor you will see that it does not stay in the same place. Water flows from one place to another place. Again, if the cap of a ballon full of gas is open gas rapidly spreads all over. We cannot easily understand whether gas has spread or not. But if ballon containing H2S or bad odour gas is open, then it spreads quickly around and you get a very bad smell. From this we understand that gas has traveled from one place to another place. Flow of fluid from one place to another place is called flow of fluid.
Fluid Flow is a part of fluid mechanics and deals with fluid dynamics. A fluid such as gases and liquids in motion is called as fluid flow. If fluid is incompressible and if there is no viscosity then it is called ideal fluid. Practically all liquids are incompressible, so liquid behaves as fluid. Gas is a compressible fluid. Because, gas can easily be forced to confine in a closed container. Flow of fluids can be steady or non-steady. It the velocity of the fluid at any point does not change with time, then that flow of fluid is called steady motion.
For example slow motion of current. On the other hand if velocity of the fluid is time dependent i.e., changes from time to time and changes from point to point then that flow is called unsteady or turbulent. For example, flow of water in the sea. Flow can either be viscous or non-viscous; viscosity of fluid is similar to the property of frictional motion of a body.