what is the difference between servo motor and steper motor

A servo motor is controlled by counting light pulses inside the servo. A light inside the servo shines on a disk with slits in it and a photocell recieves the light through the slits as the motor rotates. The photocell counts the flashes of light and sends that signal to the motor controller. The controller counts the pulses and adjusts the speed based on the operators required inputs. A stepper motor is controlled by counting magnetic pulses from a magnet and pickup mounted on a motor. A controller counts the pulses and adjusts the speed based on the operators required inputs. 

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The basic difference between a traditional stepper and a servo-based system is the type of motor and how it is controlled. Steppers typically use50 to100 pole brushless motors while typical servo motors have only4 to12 poles. A pole is an area of a motor where a North or South magnetic pole is generated either by a permanet magnet or by passing current through the coils of a winding.

Steppers don't require encoders since they can accurately move between their many poles whereas servos, with few poles, require an encoder to keep track of their position. Steppers simply move incrementally using pulses [open loop] while servo's read the difference between the motors encoder and the commanded position [closed loop], and adjust the current required to move.

Servo motors is either an ac or dc motor using encoder feedback for positioning a stepper motor is a hybrid motor moving from pole to pole in a degree step and costs less than servo motors no encoders are needed for steper motors but are sometimes fitted for closed loop feedback

shortly the stepper motor is a type of servomotor.
They are not very large and are easy to control because you can simply count steps.
and servo motor is any motorized installation where you can control position.
Any motor can be made to work as a servo motor, provided it has a positioning control system

1-Cost The cost for a servo motor and servo motor system is higher than that of a stepper motor system with equal power rating.
This feature would have to go to stepper motors.
Steppers are generally cheaper than servo motors that have the same power rating.
2-Versatility Servo motors are very versatile in their use for automation and CNC applications.
Stepper motors are also very versatile in their use for automation and CNC applications Because of their simplicity stepper motors may be found on anything from printers to clocks.
3-Reliability This is a toss up because it depends on the environment and how well the motor is protected.
The stepper takes this category only because it does not require an encoder which may fail.
4-Flexibility in motor resolution Since the encoder on a servo motor determines the motor resolution servos have a wide range of resolutions available Stepper motors usually have 1.8 or 0.9 degree resolution.
However thanks to micro-stepping steppers can obtain higher resolutions.
This is up to the driver and not the motor.
5-Power Range Because servo motors are available in DC and AC servo motors have a very wide power availability range.
The power availability range for stepper motors is not that of servo motors.