In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of strain rate. A consequence of this behavior is Pascals law which describes the role of pressure in characterizing a fluids state. 
The behavior of fluids can be described by the Navier–Stokes equations—a set of partial differential equations which are based on:
* continuity (conservation of mass),
* conservation of linear momentum,
* conservation of angular momentum,
* conservation of energy.
The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.