Ok im not an expert, but servos are normally motors that can move to a certain position depending on the width of the pulses being sent to it.
It does this using a potentiometer, connected to the output shaft, which changes its resistance depending on how far it has been rotated. Its resistance is measured by the circuit in the servo and thats how it knows where to position the output shaft.
To get continuous rotation, the potentiometer has to be disconnected from the output shaft, so that it no longer rotates when the servo is operating. This means that if you tell it to go to a position other than centre, it will rotate continuously trying to get to that position.
There is a tutorial on this site that shows you how to modify one for continuous rotation, but it doesnt say to modify it in the tutorial. I dont know why?
As i say, i myself am a beginner and may be wrong