As I remember this from the old days; you would find 'synchro's' in a typical control chain such as a positioning system, not dissimilar to a gun turret (ex-navy that I am) or some other device that you wish to control remotely.
CX -> CDX -> CT
where the CX is the Control Transmitter, the CDX is the Control Differential Trasmitter (EMD or Error Measuring Device) and the CT is the Control Transformer.
This the closed loop system where you 'turn the knob' say 30 degrees and the business end will move 30 degrees as the magnetic fields align themselves.
I suggest googling Control Sychro Chains or something similar for a far better explanation than I can give here.
Servo's you will find in applications such as 'fly by wire' on an aircraft (flaps, rudder etc) and are not totally dissimilar but offer more torque (if I remember ???).