Fluid mechanics is the branch of physics concerned with the mechanics of fluids and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and biomedical engineering, geophysics, oceanography, meteorology, astrophysics, and biology. (Source: Wikipedia)

Example of fluid mehanics in action:  Waste flowing through the sewer system, gases moving through an engine, or sap moving sucrose from the leaves to the distal parts of a tree, or even cars moving through the traffic grid.