- A rotary encoder is a sensor or transducer for converting rotary motion or position to a series of electronic pulses.
- Likewise, a linear encoder is a sensor or transducer for converting linear motion or position to a series of electronic pulses. Linear encoder technologies include capacitive, inductive, eddy current, magnetic, and optical. Optical technologies include shadow, self imaging and interferometric. Linear encoders are used in metrology instruments and high precision machining tools ranging from digital calipers to coordinate measuring machines
An encoder is a device used to encode a signal (such as a bitstream) or data into a form that is acceptable for transmission or storage. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analogue encoding is done with analog circuitry.

See also
External links
asic-world's Digital Combinational Logic (part III) - a very good overview of encoders.