An encoder is a feedback device that is primarily used to reference position of a certain object. They can be linear or rotary. An example of a linear encoder would be a linear scale commonly used on different machine axis to reference the position of the axis to correspond with the machine program. Rotary encoders are commonly used on servo and spindle motors. On a servo motor they are used to detect shaft/rotor position in reference to the stator so that the drive running the motor knows which phase to fire next (commutation). On a spindle motor an encoder can also detect velocity and reference back to the controller to keep speed stablilized.
Encoders can communicate using a variety of different languages (so to say). The most common is the incremental encoder. There are also absolute, serial, TTL, 1Vp-p, 11 uAmp, All pretty much do the same thing, they count. Encoders can also come in various different counts from as low as 500 PPR (Pulse Per Revolution) up to in excess of 1,000,000 PPR. So, depending on how accurate you need to be there should be an encoder out there to fit your needs.
If you are interested in an article about some problems with encoders here is a link to an article
http://www.repairzone.com/learningce...o-motor-repair
Jade