Actuators are the devices that move the control
surfaces on an aircraft; they are installed into the overall control circuit in
one of two ways - either connected in parallel or in series. When connected in
parallel they allow the control column to follow the movement of the control
surface (ie if you bank to starboard the control column will reflect that).
On the other hand, most of the time the control
surfaces are also moving very slightly to counter minor deviations in flight
path, and under these conditions it would be undesirable for the control column
to shuffle around continuously. A series connected actuator is therefore made
to be very stiff at the control column end in order to negate any column
movement. Large modern aircraft have both parallel and series actuators fitted.