What is a difference between encoder and tachometer?

An encoder is a device, circuit, transducer, software program or person that converts information from one format or code to another.

A tachometer is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. Most commonly this is the little rev counter on your car/truck that shows you the rate of rotation of the engine's crankshaft. Or in layman's terms, the RPM gauge.

Hope this helps