What is the diff between Stepper & Servo motors

Stepper motors move a fixed angle per step (unless they are overloaded, and then they just vibrate). Servos move any amount based on a feedback device that measures their position and reports back to the control circuit. They can also be overloaded, but at least the control circuit knows that they are not going to where they are supposed to be.
With a smooth enough floor and round enough wheels and bearings, you can move the space shuttle with two 4 volt .9 amp motors. With an ordinary floor and Rc car wheels, you can move 20 pounds if you gear the motors low enough. The motors have to overcome rolling friction, air drag, and also have some force to spare to produce acceleration. This is a very poorly defined problem as stated. For some actual load and wheel situation, you probably need to do some experimenting to better estimate the motive power required.

I assume you know how steppers work. Servo motors differ in that they are basically standard armature type, either brushed PM or brushless motors. They are necessarily feedback devices, using an encoder to provide closed loop operation and position ref. Steppers can be open loop, with no position ref from the motor itself. Steppers have great low rpm torque, low max rpm. Good for direct drive positioning. Geared servos (think R/C servos) are good for your steering application. JR zalz>

Actually, a servo motor is not a kind of motor, but a feedback control system that contain any sort of motor. I have seen servo systems made with stepper, synchronous and induction motors, as well as with shunt wound and permanent magnet motors. Somebody has probably made servos out of series wound motors also, but stabilizing the control loop is a bit harder because of the torque proportional to current squared property of this type.

------------- A stepper steps, a servo servoes. Servos seek a position that is signalled to them by their data input, analog or digital, whereas a servo can be made with a stepper or with a brush motor.
------------------------- Not necessarily.
------------ Where?
--------------- Slowly enough, and with good enough bearings, anything can move anything.
-Steve

Shouldn't that be "serves"?

Yes, in the general sense you're right.
"Servo" referrs to a feedback technique for controlling position.
"Servo Mechanism" referrs to any mechanism using this technique.
"Servo Motor" referrs to any motor using it.
However, in hobby robotics, "servo" is generally used to mean an R/C Servo Motor, rather than the more general concepts. -- D. Jay Newman

The shortest answer is that they are completely different and have nothing to do with each other, and shouldn't be confused.
A servo is more than a motor: it's both a motor and a feedback mechanism. You tell a servo "I want you to move to the following position" and the servo just plain does it. Commercial servoes typically have a fixed range of motion; they can turn their output shaft something like 270 degrees lock to lock. This makes them good for things like steering; you just say "go to straight ahead" and quit worrying about it.
A stepper motor is, in some respects, the exact opposite of a servo. You tell a stepper motor you want it to move a step, and it does so. Keeping track of how many steps it's gone, and details like not giving it too many steps too quickly, are up to you. A stepper can keep going round and round, which is why it's good for drive wheels.
Notice that you can do a servo in software by using a stepper, and counting how many steps you've given it.
Nowhere near enough information. How much torque do the motors provide? How are they geared? What sort of acceleration do you want?

Thanks guys for clearing that up. Its much clearer now.
One more question :
What determines torque or turning power of a stepper motor - voltage or current?
i.e. All other things being equal, will a 3 volt, 0.35 A stepper motor deliver more torque than say a 5.3 volt, 0.17A motor?

Say u were to tell the servo motor (mechanism) to spin half way around and i were to lightly grip the wheel to give it resistance. Would the circuitry detect that and transfer more current to the motor to make it overcome the resistance and spin to the desired location?

Yes. That is what the feedback signal accomplishes. The motor receives just the current it needs to make the desired move.

All torque comes from the interaction of stator and rotor magnetic fields. And magnetic fields are proportional to the current that causes them (if they come from a coil and not a permanent magnet). So torque is proportional to current (if the current energizes one electromagnet) or current squared (in the case of a series wound motor, where the same current magnetizes both the rotating part and the stationary part, in series).
But it takes voltage to cause current to happen. And the faster a motor is turning, the more voltage it generates. The applied voltage must overcome the generated voltage before any current can be moved through the motor. So voltage must be available (and more for higher speed) but that voltage supply must have the ability to deliver current to produce torque. Total power is proportional to volts times amps (and to torque times speed).