Question 1: Can't servos only spin as far as the attached pot allows? If you have answered that: Question 2: I have played around with the Lego NXT robot, and the servo motors there are capable of infinite spin. In programs, you can, and I have, say, "Go forward for 698 degrees." And for those of you who can't do math, that is almost 2 full rotations. So how is the motor able to spin freely AND be controlled in a servo-like way? As I asked before, aren't servos restricted in their movement?
A servo can only travel the distance the mechanically allow (how far the pot and gears can rotate)
If it can do infinitive spin, it is either a modified servo that got no idea of where it is, or if you can tell it exactly how far to go, it is probably actually a stepper motor.
Could be a DC motor with an encoder instead of a pot. There must be details out there, surely? Just because it is called a servo, doesn't mean it has to be constructed like an R/C servo.
NXT "servos" are just dc motors with rotary encoders, controlled through I2C, I believe.
A Continuous Rotation Servo is a horrible abomination. There, I said it.
A SERVO, as Richard pointed out, is about feedback. No feed back, not a SERVO anymore. What you have is a geared motor that can be speed controlled with the RC servo control specification. But again... what you have is a variable speed motor with no feedback and therefore no control over accuracy of position.
Stepper motor interfacing is "Haaaaard" compared to this, so I can see why a C.R.S. is often used as an alternative to a stepper motor. (I said "HARD" in my emo voice)
As said.. the NXT stuff is totally unique to LEGO.. but since it has feedback... you could get away with calling it a SERVO... just not an R/C SERVO.