When we connect the resistances in series , we must know that the current is constant when we move at the same line , and the voltage will be change when we move from resistance to another , and every resistance have voltage different values , from the previous we can write this equation :
when we connect resistance at parallel we must calculate the equivalent resistance , and to do this we must know some properties about this state , firstly the voltage is constant between any two point in the circuit , so when we change the wire fore example from the wire which R1 in it ro the wire which R2 in it the voltage keep constant and 
the electric circuits contains some important parts and the electric circuits can't work without it , the first part is the voltage source , this part is the main part , and it give the electric circuits the voltage , the 2nd part is the resistances and this the part which we want to use it contain any machine we want to use it , and all of thing which work by electric current , as lamp for example , the 3rd part is the conductors and it mean the wires , and 4th part is the key , by this part we can switch the electric circuits on or off .
At this picture we see cylinders connected parallel , we use this system to move poth cylinders at the same time when the oil reach the lever , and we move the lever to the first state ( from the pump to the cylinder and then the oil return to the tank ) whech shown at the up square , the oil start to flow to the cylinders then the pressure cause to push the pistons , by this movement we can move the load up or down , the oil which existing at the other side of the cylinder start to flow and goes back to the tank , then we change the lever to the state which show in the down square of the lever picture which allow to the oil which existing in the left side of the cylinder to go back to the tank and the oil flow to reach the other side ( right side ) we now move the load down and it return to the tank .
We have at this simple hydrolic system 2 piston and liquid between the two piston , we alway use this system to rise load up , when we want to do this we put the load on the piston which has the biggest area , we make a difference between the areas of the pistons , to get high force which will be more than the force which we can get it by hand or leg by human , when we start to down the left piston by hand or leg , the pressure on the piston increase and on the other hand the pressure must be equal to left side , this process result to rise the load up because the area on the other hand is biggest than the left , and by the low ( pressure = force / area ) we get suitable force to rise the load up .
The oil start to flow through the input which existing at the left side and when the oil increase at the left side the pressure will increase at this side , result of this pressure the piston ( plunger at this picture ) start to move to the other side and this movement will rise up the load which we want to rise it up , we now get what we want , but for repeat this process we must down the piston as it was at the starting state , to do this the oil start to flow through the right side and the pressure will increase at this side which result to push the piston to left and this movement will return the piston to the starting state which allow to us to use it again to rise another load .