For fluid systems we have to conserved momentum, as momentum is neither created nor destroyed, but only changed through the action of forces as described by Newton's laws of motion. The momentum balance can be generated from the general continuity equation by the use of the mass flux, this generates the Cauchy momentum equation. This equation can then be simplified to produce the very useful Bernoulli's equation.
The video below shows the derivation and then simplification of the general continuity balance to form Bernoulli's equation:
The interactive graph below lets you explore the Bernoulli equation for an ideal frictionless fluid.