This can get very complicated, so we'll focus on one simple case, but we should briefly mention the different categories of fluid flow.
Fluids can flow steadily, or be turbulent.
In steady flow, the fluid passing a given point maintains a steady velocity. For turbulent flow, the speed and or the direction of the flow varies. In steady flow, the motion can be represented with streamlines showing the direction the water flows in different areas. The density of the streamlines increases as the velocity increases. Fluids can be compressible or incompressible.