excuse me, what direction should i take?

We talked about what creates the flow in a hydraulic system, now I would like to get into what controls that flow.  A valve is the device that controls the direction, pressure, or the speed of the hydraulic fluid.  There are several different types of valves. Hydraulic circuit valves are grouped by the function they perform. 

The route by which the fluid takes through a valve is called the way. There are one, two, three and four way valves A pressure relief valve is a one way valve. When the pressure starts to get to high in the circuit the fluid pushes against a poppet inside the valve. This poppet is connected to a spring which intern gets depressed and opens up a port for the fluid to flow through thus reducing the pressure in the circuit.

an example of a electrical
directional control valve.

Directional valves control the direction of the fluid through the circuit. These can be two, three, or four way valves. The valves have an internal spool that changes position thus opening and closing certain pathways for the fluid to flow though. The mechanism to change the position of the spool can be manual, mechanical, hydraulic, or electric. The most common mechanism are electric solenoids.