The basic principle of hydraulics is put forth by Pascal's Law, which states that any pressure applied to a fluid is transmitted equally through the fluid, such as to the walls of the container. Hydraulic systems typically consist of a hydraulic fluid that transmits pressure applied at one point to other points in the system, a piston that initiates the pressure, a reservoir to contain the hydraulic fluid and a mechanism that fluid pressure actuates to perform work.
For example, when someone presses a car brake, the brake fluid is the hydraulic fluid, his foot and the adjacent brake pedal mechanism is the piston, the refillable brake fluid container beneath the hood and the brake fluid lines constitute the reservoir and the brake pads are the actuating mechanism, since they press upon the wheels, slowing the vehicle. A tube of toothpaste is a hydraulic system that lacks an actuating mechanism; a person's fingers are the pistons, the toothpaste is the fluid, the tube is the reservoir and, because there is no actuating mechanism, the toothpaste simply oozes out of the end, performing no work.
Hydraulics derives its name from "hydro," meaning water, and is concerned with liquids as opposed to other types of fluids, such as gases or plasmas. Therefore closely related principles, such as basic principles about gases expounded by Bernoulli's Law, are not the province of hydraulics but rather are more properly the province of fluid dynamics.